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<chapter id="jpa_overview_mapping">
    <title>
        Mapping Metadata
    </title>
    <indexterm zone="jpa_overview_mapping">
        <primary>
            mapping metadata
        </primary>
    </indexterm>
    <indexterm>
        <primary>
            entities
        </primary>
        <secondary>
            mapping to database
        </secondary>
        <see>
            mapping metadata
        </see>
    </indexterm>
    <indexterm>
        <primary>
            metadata
        </primary>
        <secondary>
            mapping metadata
        </secondary>
        <see>
            mapping metadata
        </see>
    </indexterm>
    <indexterm>
        <primary>
            ORM
        </primary>
        <seealso>
            mapping metadata
        </seealso>
    </indexterm>
    <indexterm zone="jpa_overview_mapping">
        <primary>
            JPA
        </primary>
        <secondary>
            object-relational mapping
        </secondary>
        <seealso>
            mapping metadata
        </seealso>
    </indexterm>
    <para>
<emphasis>Object-relational mapping</emphasis> is the process of mapping
entities to relational database tables. In JPA, you perform
object/relational mapping through <emphasis>mapping metadata</emphasis>.
Mapping metadata uses annotations to describe how to link your object model to
your relational model.
    </para>
    <note>
        <para>
OpenJPA offers tools to automate mapping and schema creation. See
<xref linkend="ref_guide_mapping"/> in the Reference Guide.
        </para>
    </note>
    <para>
Throughout this chapter, we will draw on the object model introduced in
<xref linkend="jpa_overview_meta"/>. We present that model again below.
As we discuss various aspects of mapping metadata, we will zoom in on specific
areas of the model and show how we map the object layer to the relational layer.
    </para>
    <mediaobject>
        <imageobject>
            <!-- PNG image data, 553 x 580 (see README) -->
            <imagedata fileref="img/jpa-meta-model.png" width="369px"/>
            
        </imageobject>
    </mediaobject>
    <para>
All mapping metadata is optional. Where no explicit mapping metadata is given,
JPA uses the defaults defined by the specification. As we present
each mapping throughout this chapter, we also describe the defaults that apply
when the mapping is absent.
    </para>
    <note>
        <para>
Mapping metadata is used primarily with schema generation. This metadata should not
be relied upon for validation prior to communicating with the database. 
For example using the @Column(nullable=false) annotation does not do up front validation
that the value in the entity is correct. 
        </para>
    </note>
    <section id="jpa_overview_mapping_table">
        <title>
            Table
        </title>
        <indexterm zone="jpa_overview_mapping_table">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                class
            </secondary>
            <tertiary>
                table attribute
            </tertiary>
        </indexterm>
        <para>
The <classname>Table</classname> annotation specifies the table for an entity
class. If you omit the <classname>Table</classname> annotation, base entity
classes default to a table with their unqualified class name. The default table
of an entity subclass depends on the inheritance strategy, as you will see in
<xref linkend="jpa_overview_mapping_inher"/>.
        </para>
        <para>
<classname>Table</classname>s have the following properties:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>String name</literal>: The name of the table. Defaults to the
unqualified entity class name.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>String schema</literal>: The table's schema. If you do not name a
schema, JPA uses the default schema for the database connection.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>String catalog</literal>: The table's catalog. If you do not name a
catalog, JPA uses the default catalog for the database connection.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>UniqueConstraint[] uniqueConstraints</literal>: An array of unique
constraints to place on the table. We cover unique constraints below. Defaults
to an empty array.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The equivalent XML element is <literal>table</literal>. It has the following
attributes, which correspond to the annotation properties above:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>name</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>schema</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>catalog</literal>
                </para>
            </listitem>
        </itemizedlist>
        <para>
The <literal>table</literal> element also accepts nested <literal>
unique-constraint</literal> elements representing unique constraints. We will
detail unique constraints shortly.
        </para>
        <para>
Sometimes, some of the fields in a class are mapped to secondary tables. In that
case, use the class' <classname>Table</classname> annotation to name what you
consider the class' primary table. Later, we will see how to map certain fields
to other tables.
        </para>
        <para>
The example below maps classes to tables to separate schemas. The
<literal>CONTRACT</literal>, <literal>SUB</literal>, and <literal>LINE_ITEM
</literal> tables are in the <literal>CNTRCT</literal> schema; all other tables
are in the default schema.
        </para>
        <example id="jpa_overview_mapping_classex">
            <title>
                Mapping Classes
            </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
public class Magazine {
    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART")
public class Article {
    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {
    ...
}

@Entity
@Table(name="AUTH")
public class Author {
    ...
}

@Embeddable
public class Address {
    ...
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {
    ...
}

@Entity
@Table(schema="CNTRCT")
public class Contract
    extends Document {
    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
public class Subscription {
    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {
        ...
    }
}

@Entity(name="Lifetime")
public class LifetimeSubscription
    extends Subscription {
    ...
}

@Entity(name="Trial")
public class TrialSubscription
    extends Subscription {
    ...
}
</programlisting>
            <para>
The same mapping information expressed in XML:
            </para>
<programlisting>
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
    version="1.0"&gt;
    &lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
        ...
    &lt;/mapped-superclass&gt;
    &lt;entity class="org.mag.Magazine"&gt;
        &lt;table name="MAG"/&gt;
        &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.Article"&gt;
        &lt;table name="ART"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Company"&gt;
        &lt;table name="COMP"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Author"&gt;
        &lt;table name="AUTH"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Contract"&gt;
        &lt;table schema="CNTRCT"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Subscription"&gt;
        &lt;table name="SUB" schema="CNTRCT"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
        &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
        ...
    &lt;/entity&gt;
    &lt;embeddable class="org.mag.pub.Address"&gt;
        ...
    &lt;/embeddable&gt;
&lt;/entity-mappings&gt;
</programlisting>
        </example>
    </section>
    <section id="jpa_overview_mapping_unq">
        <title>
            Unique Constraints
        </title>
        <indexterm zone="jpa_overview_mapping_unq">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                unique constraints
            </secondary>
            <seealso>
                unique constraints
            </seealso>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_unq">
            <primary>
                unique constraints
            </primary>
        </indexterm>
        <para>
Unique constraints ensure that the data in a column or combination of columns is
unique for each row. A table's primary key, for example, functions as an
implicit unique constraint. In JPA, you represent other unique
constraints with an array of <classname> UniqueConstraint</classname>
annotations within the table annotation. The unique constraints you define are
used during table creation to generate the proper database constraints, and may
also be used at runtime to order <literal>INSERT</literal>, <literal>UPDATE
</literal>, and <literal>DELETE</literal> statements. For example, suppose there
is a unique constraint on the columns of field <literal>F</literal>. In the
same transaction, you remove an object <literal>A</literal> and persist a new
object <literal>B</literal>, both with the same <literal>F</literal> value. The
JPA runtime must ensure that the SQL deleting <literal>A</literal>
is sent to the database before the SQL inserting <literal>B</literal> to avoid a
unique constraint violation.
        </para>
        <para>
<classname>UniqueConstraint</classname> has these properties:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>String name</literal>: The name of the constraint. OpenJPA will choose
a name if you do not provide one, or will create an anonymous constraint.
                </para>
            </listitem>            
            <listitem>
                <para>
<literal>String[] columnNames</literal>: The names of the columns the
constraint spans.
                </para>
            </listitem>
        </itemizedlist>
        <para>
In XML, unique constraints are represented by nesting <literal>
unique-constraint</literal> elements within the <literal> table</literal>
element. Each <literal>unique-constraint</literal> element in turn nests
<literal>column-name</literal> text elements to enumerate the constraint's
columns.
        </para>
        <example id="jpa_overview_mapping_unq_attrex">
            <title>
                Defining a Unique Constraint
            </title>
            <para>
The following defines a unique constraint on the <literal> TITLE</literal>
column of the <literal>ART</literal> table:
            </para>
<programlisting>
@Entity
@Table(name="ART", uniqueConstraints=@UniqueConstraint(name="TITLE_CNSTR", columnNames="TITLE"))
public class Article {
    ...
}
</programlisting>
            <para>
The same metadata expressed in XML form:
            </para>
<programlisting>
&lt;entity class="org.mag.Article"&gt;
    &lt;table name="ART"&gt;
        &lt;unique-constraint&gt;
            &lt;name&gt;TITLE_CNSTR&lt;/name&gt;
            &lt;column-name&gt;TITLE&lt;/column-name&gt;
        &lt;/unique-constraint&gt;
    &lt;/table&gt;
    ...
&lt;/entity&gt;
</programlisting>
        </example>
    </section>
    <section id="jpa_overview_mapping_column">
        <title>
            Column
        </title>
        <indexterm zone="jpa_overview_mapping_column">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                Column
            </secondary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_column">
            <primary>
                Column
            </primary>
            <secondary>
                in mapping metadata
            </secondary>
            <seealso>
                mapping metadata
            </seealso>
        </indexterm>
        <para>
In the previous section, we saw that a <classname>UniqueConstraint</classname>
uses an array of column names. Field mappings, however, use full-fledged
<classname>Column</classname> annotations. Column annotations have the following
properties:
        </para>
        <itemizedlist>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        name property
                    </tertiary>
                </indexterm>
<literal>String name</literal>: The column name. Defaults to the field name.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        columnDefinition property
                    </tertiary>
                </indexterm>
<literal>String columnDefinition</literal>: The database-specific column type
name. This property is only used by vendors that support creating tables from
your mapping metadata. During table creation, the vendor will use the value of
the <literal>columnDefinition</literal> as the declared column type. If no
<literal>columnDefinition</literal> is given, the vendor will choose an
appropriate default based on the field type combined with the column's length,
precision, and scale.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        length property
                    </tertiary>
                </indexterm>
<literal>int length</literal>: The column length. This property is typically
only used during table creation, though some vendors might use it to validate
data before flushing. <literal>CHAR</literal> and <literal>VARCHAR</literal>
columns typically default to a length of 255; other column types use the
database default.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        precision property
                    </tertiary>
                </indexterm>
<literal>int precision</literal>: The precision of a numeric column. This
property is often used in conjunction with <literal>scale</literal> to form the
proper column type name during table creation.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        scale property
                    </tertiary>
                </indexterm>
<literal>int scale</literal>: The number of decimal digits a numeric column can
hold. This property is often used in conjunction with <literal>precision
</literal> to form the proper column type name during table creation.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        nullable property
                    </tertiary>
                </indexterm>
<literal>boolean nullable</literal>: Whether the column can store null values.
Vendors may use this property both for table creation and at runtime; however,
it is never required. Defaults to <literal>true</literal>.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        insertable property
                    </tertiary>
                </indexterm>
<literal>boolean insertable</literal>: By setting this property to <literal>
false</literal>, you can omit the column from SQL <literal>INSERT</literal>
statements. Defaults to <literal>true</literal>.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        updatable property
                    </tertiary>
                </indexterm>
<literal>boolean updatable</literal>: By setting this property to <literal>
false</literal>, you can omit the column from SQL <literal>UPDATE</literal>
statements. Defaults to <literal>true</literal>.
                </para>
            </listitem>
            <listitem>
                <para>
                <indexterm>
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        Column
                    </secondary>
                    <tertiary>
                        table property
                    </tertiary>
                </indexterm>
<literal>String table</literal>: Sometimes you will need to map fields to
tables other than the primary table. This property allows you specify that the
column resides in a secondary table. We will see how to map fields to secondary
tables later in the chapter.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The equivalent XML element is <literal>column</literal>. This element has
attributes that are exactly equivalent to the <classname> Column</classname>
annotation's properties described above:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>name</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>column-definition</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>length</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>precision</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>scale</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>insertable</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>updatable</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>table</literal>
                </para>
            </listitem>
        </itemizedlist>
    </section>
    <section id="jpa_overview_mapping_id">
        <title>
            Identity Mapping
        </title>
        <indexterm zone="jpa_overview_mapping_id">
            <primary>
                Id
            </primary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_id">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                identity
            </secondary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_id">
            <primary>
                identity
            </primary>
            <secondary>
                mapping
            </secondary>
        </indexterm>
        <para>
With our new knowledge of columns, we can map the identity fields of our
entities. The diagram below now includes primary key columns for our model's
tables. The primary key column for <classname>Author</classname> uses
nonstandard type <literal> INTEGER64</literal>, and the <literal>Magazine.isbn
</literal> field is mapped to a <literal>VARCHAR(9)</literal> column instead of
a <literal>VARCHAR(255)</literal> column, which is the default for string
fields. We do not need to point out either one of these oddities to the JPA
implementation for runtime use. If, however, we want to use the JPA
implementation to create our tables for us, it needs to know about
any desired non-default column types. Therefore, the example following the
diagram includes this data in its encoding of our mappings.
        </para>
        <mediaobject>
            <imageobject>
                <!-- PNG image data, 513 x 410 (see README) -->
                <imagedata fileref="img/jpa-mapping-identity.png" width="341px"/>
                
            </imageobject>
        </mediaobject>
        <para>
Note that many of our identity fields do not need to specify column information,
because they use the default column name and type.
        </para>
        <example id="jpa_overview_mapping_identityex">
            <title>
                Identity Mapping
            </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
public class Article {

    @Id private long id;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Column(name="AID", columnDefinition="INTEGER64")
    @Id private long id;

    ...
}

@Embeddable
public class Address {
    ...
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...
}

@Entity
@Table(schema="CNTRCT")
public class Contract
    extends Document {
    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
public class Subscription {

    @Id private long id;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {
        ...
    }
}

@Entity(name="Lifetime")
public class LifetimeSubscription
    extends Subscription {
    ...
}

@Entity(name="Trial")
public class TrialSubscription
    extends Subscription {
    ...
}
</programlisting>
            <para>
The same metadata for <literal>Magazine</literal> and <literal>Company</literal>
expressed in XML form:
            </para>
<programlisting>
&lt;entity class="org.mag.Magazine"&gt;
    &lt;id-class class="org.mag.Magazine.Magazine.MagazineId"/&gt;
    &lt;table name="MAG"/&gt;
    &lt;attributes&gt;
        &lt;id name="isbn"&gt;
            &lt;column length="9"/&gt;
        &lt;/id&gt;
        &lt;id name="title"/&gt;
      ...  
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Company"&gt;
    &lt;table name="COMP"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"&gt;
            &lt;column name="CID"/&gt;
        &lt;/id&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
        </example>
    </section>
    <section id="jpa_overview_mapping_sequence">
        <title>
            Generators
        </title>
        <indexterm zone="jpa_overview_mapping_sequence">
            <primary>
                generators
            </primary>
            <secondary>
                mapping metadata
            </secondary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_sequence">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                generators
            </secondary>
            <seealso>
                TableGenerator
            </seealso>
            <seealso>
                SequenceGenerator
            </seealso>
        </indexterm>
        <para>
One aspect of identity mapping not covered in the previous section is JPA's
ability to automatically assign a value to your numeric identity fields using
<emphasis>generators</emphasis>. We discussed the available generator types in
<xref linkend="jpa_overview_meta_id"/>. Now we show you how to define
named generators.
        </para>
        <section id="jpa_overview_mapping_sequence_seqgen">
            <title>
                Sequence Generator
            </title>
            <indexterm zone="jpa_overview_mapping_sequence_seqgen">
                <primary>
                    generators
                </primary>
                <secondary>
                    SequenceGenerator
                </secondary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_sequence_seqgen">
                <primary>
                    SequenceGenerator
                </primary>
            </indexterm>
            <para>
Most databases allow you to create native sequences. These are database
structures that generate increasing numeric values. The <classname>
SequenceGenerator</classname> annotation represents a named database sequence.
You can place the annotation on any package, entity class, persistent field
declaration (if your entity uses field access), or getter method for a
persistent property (if your entity uses property access). <classname>
SequenceGenerator</classname> has the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            SequenceGenerator
                        </primary>
                        <secondary>
                            name property
                        </secondary>
                    </indexterm>
<literal>String name</literal>: The generator name. This property is required.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            SequenceGenerator
                        </primary>
                        <secondary>
                            sequenceName property
                        </secondary>
                    </indexterm>
<literal>String sequenceName</literal>: The name of the database sequence. If
you do not specify the database sequence, your vendor will choose an appropriate
default.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            SequenceGenerator
                        </primary>
                        <secondary>
                            initialValue property
                        </secondary>
                    </indexterm>
<literal>int initialValue</literal>: The initial sequence value.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            SequenceGenerator
                        </primary>
                        <secondary>
                            allocationSize property
                        </secondary>
                    </indexterm>
<literal>int allocationSize</literal>: The number of values to allocate in
memory for each trip to the database. Allocating values in memory allows the JPA
runtime to avoid accessing the database for every sequence request.
This number also specifies the amount that the sequence value is incremented
each time the sequence is accessed. Defaults to 50.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            SequenceGenerator
                        </primary>
                        <secondary>
                            schema property
                        </secondary>
                    </indexterm>
                <literal>String schema</literal>: The sequence's schema. If you do not name a
schema, JPA uses the default schema for the database connection.
                    </para>
                </listitem>
            </itemizedlist>
            <note>
                <para>
OpenJPA allows you to use one of OpenJPA's built-in generator
implementations in the <literal>sequenceName</literal> property. You can also
set the <literal>sequenceName</literal> to <literal>system</literal> to use the
system sequence defined by the <link linkend="openjpa.Sequence"><literal>
openjpa.Sequence</literal></link> configuration property. See the Reference
Guide's <xref linkend="ref_guide_sequence"/> for details.
                </para>
            </note>
            <para>
The XML element for a sequence generator is <literal>sequence-generator
</literal>. Its attributes mirror the above annotation's properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>sequence-name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>initial-value</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>allocation-size</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>schema</literal>
                    </para>
                </listitem>
            </itemizedlist>
            <para>
To use a sequence generator, set your <classname>GeneratedValue</classname>
annotation's <literal>strategy</literal> property to <literal>
GenerationType.SEQUENCE</literal>, and its <literal>generator</literal> property
to the sequence generator's declared name. Or equivalently, set your <literal>
generated-value</literal> XML element's <literal>strategy</literal> attribute to
<literal>SEQUENCE</literal> and its <literal>generator</literal> attribute to
the generator name.
            </para>
        </section>
        <section id="jpa_overview_mapping_sequence_tablegen">
            <title>
                Table Generator
            </title>
            <indexterm zone="jpa_overview_mapping_sequence_tablegen">
                <primary>
                    generators
                </primary>
                <secondary>
                    TableGenerator
                </secondary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_sequence_tablegen">
                <primary>
                    TableGenerator
                </primary>
            </indexterm>
            <para>
A <classname>TableGenerator</classname> refers to a database table used to store
increasing sequence values for one or more entities. As with <classname>
SequenceGenerator</classname>, you can place the <classname>TableGenerator
</classname> annotation on any package, entity class, persistent field
declaration (if your entity uses field access), or getter method for a
persistent property (if your entity uses property access). <classname>
TableGenerator</classname> has the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            name property
                        </secondary>
                    </indexterm>
<literal>String name</literal>: The generator name. This property is required.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            table property
                        </secondary>
                    </indexterm>
<literal>String table</literal>: The name of the generator table. If left
unspecified, your vendor will choose a default table.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            schema property
                        </secondary>
                    </indexterm>
<literal>String schema</literal>: The named table's schema.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            catalog property
                        </secondary>
                    </indexterm>
<literal>String catalog</literal>: The named table's catalog.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            pkColumnName property
                        </secondary>
                    </indexterm>
<literal>String pkColumnName</literal>: The name of the primary key column in
the generator table. If unspecified, your implementation will choose a default.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            valueColumnName property
                        </secondary>
                    </indexterm>
<literal>String valueColumnName</literal>: The name of the column that holds
the sequence value. If unspecified, your implementation will choose a default.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            pkColumnValue property
                        </secondary>
                    </indexterm>
<literal>String pkColumnValue</literal>: The primary key column value of the
row in the generator table holding this sequence value. You can use the same
generator table for multiple logical sequences by supplying different <literal>
pkColumnValue</literal> s. If you do not specify a value, the implementation
will supply a default.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            initialValue property
                        </secondary>
                    </indexterm>
<literal>int initialValue</literal>: The value of the generator's first issued
number.
                    </para>
                </listitem>
                <listitem>
                    <para>
                    <indexterm>
                        <primary>
                            TableGenerator
                        </primary>
                        <secondary>
                            allocationSize property
                        </secondary>
                    </indexterm>
<literal>int allocationSize</literal>: The number of values to allocate in
memory for each trip to the database. Allocating values in memory allows the JPA
runtime to avoid accessing the database for every sequence request.
This number also specifies the amount that the sequence value is incremented
each time the generator table is updated. Defaults to 50.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
The XML equivalent is the <literal>table-generator</literal> element. This
element's attributes correspond exactly to the above annotation's properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>table</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>schema</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>catalog</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>pk-column-name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>value-column-name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>pk-column-value</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>initial-value</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>allocation-size</literal>
                    </para>
                </listitem>
            </itemizedlist>
            <para>
To use a table generator, set your <classname>GeneratedValue</classname>
annotation's <literal>strategy</literal> property to <literal>
GenerationType.TABLE</literal>, and its <literal>generator</literal> property to
the table generator's declared name. Or equivalently, set your <literal>
generated-value</literal> XML element's <literal>strategy</literal> attribute to
<literal>TABLE</literal> and its <literal>generator</literal> attribute to the
generator name.
            </para>
        </section>
        <section id="jpa_overview_mapping_sequence_genex">
            <title>
                Example
            </title>
            <para>
Let's take advantage of generators in our entity model. Here are our updated
mappings.
            </para>
            <example id="jpa_overview_mapping_sequenceex">
                <title>
                    Generator Mapping
                </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq") 
    private long id;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
    @TableGenerator(name="AuthorGen", table="AUTH_GEN", pkColumnName="PK",
        valueColumnName="AID")
    @Column(name="AID", columnDefinition="INTEGER64")
    private long id;

    ...
}

@Embeddable
public class Address {
    ...
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(generate=GenerationType.IDENTITY)
    private long id;
 
    ...
}

@Entity
@Table(schema="CNTRCT")
public class Contract
    extends Document {

    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
public class Subscription {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {
        ...
    }
}

@Entity(name="Lifetime")
public class LifetimeSubscription
    extends Subscription {
    ...
}

@Entity(name="Trial")
public class TrialSubscription
    extends Subscription {
    ...
}
</programlisting>
                <para>
The same metadata for <literal>Article</literal> and <literal>Author</literal>
expressed in XML form:
                </para>
<programlisting>
&lt;entity class="org.mag.Article"&gt;
    &lt;table name="ART"&gt;
        &lt;unique-constraint&gt;
            &lt;column-name&gt;TITLE&lt;/column-name&gt;
        &lt;/unique-constraint&gt;
    &lt;/table&gt;
    &lt;sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"&gt;
            &lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
        &lt;/id&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Author"&gt;
    &lt;table name="AUTH"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"&gt;
            &lt;column name="AID" column-definition="INTEGER64"/&gt;
            &lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
            &lt;table-generator name="AuthorGen" table="AUTH_GEN" 
                pk-column-name="PK" value-column-name="AID"/&gt;
        &lt;/id&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
        </section>
    </section>
    <section id="jpa_overview_mapping_inher">
        <title>
            Inheritance
        </title>
        <indexterm zone="jpa_overview_mapping_inher">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                inheritance
            </secondary>
            <seealso>
                inheritance
            </seealso>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_inher">
            <primary>
                inheritance
            </primary>
            <secondary>
                mapping
            </secondary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_inher">
            <primary>
                entities
            </primary>
            <secondary>
                inheritance
            </secondary>
            <seealso>
                inheritance
            </seealso>
        </indexterm>
        <indexterm>
            <primary>
                impedance mismatch
            </primary>
        </indexterm>
        <para>
In the 1990's programmers coined the term <emphasis>impedance mismatch
</emphasis> to describe the difficulties in bridging the object and relational
worlds. Perhaps no feature of object modeling highlights the impedance mismatch
better than inheritance. There is no natural, efficient way to represent an
inheritance relationship in a relational database.
        </para>
        <para>
        <indexterm>
            <primary>
                mapping metadata
            </primary>
            <secondary>
                inheritance
            </secondary>
            <tertiary>
                strategy attribute
            </tertiary>
        </indexterm>
Luckily, JPA gives you a choice of inheritance strategies, making
the best of a bad situation. The base entity class defines the inheritance
strategy for the hierarchy with the <classname>Inheritance</classname>
annotation. <classname>Inheritance</classname> has the following properties:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>InheritanceType strategy</literal>: Enum value declaring the
inheritance strategy for the hierarchy. Defaults to <literal>
InheritanceType.SINGLE_TABLE</literal>. We detail each of the available
strategies below.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The corresponding XML element is <literal>inheritance</literal>, which has a
single attribute:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>strategy</literal>: One of <literal>SINGLE_TABLE</literal>, <literal>
JOINED</literal>, or <literal>TABLE_PER_CLASS</literal>.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The following sections describe JPA's standard inheritance strategies.
        </para>
        <note>
            <para>
OpenJPA allows you to vary your inheritance strategy for each class, rather than
forcing a single strategy per inheritance hierarchy. See
<xref linkend="ref_guide_mapping_jpa"/> in the Reference Guide for
details.
            </para>
        </note>
        <section id="jpa_overview_mapping_inher_single">
            <title>
                Single Table
            </title>
            <indexterm zone="jpa_overview_mapping_inher_single">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    inheritance
                </secondary>
                <tertiary>
                    SINGLE_TABLE strategy
                </tertiary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_inher_single">
                <primary>
                    inheritance
                </primary>
                <secondary>
                    SINGLE_TABLE strategy
                </secondary>
            </indexterm>
            <para>
The <literal>InheritanceType.SINGLE_TABLE</literal> strategy maps all classes in
the hierarchy to the base class' table.
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 266 x 203 (see README) -->
                    <imagedata fileref="img/inher-superclass-table.png" width="177px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
In our model, <classname>Subscription</classname> is mapped to the <literal>
CNTRCT.SUB</literal> table. <classname> LifetimeSubscription</classname>, which
extends <classname> Subscription</classname>, adds its field data to this table
as well.
            </para>
            <example id="jpa_overview_mapping_inher_singleex">
                <title>
                    Single Table Mapping
                </title>
<programlisting>
@Entity
@Table(name="SUB", schema="CNTRCT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Subscription {
    ...
}

@Entity(name="Lifetime")
public class LifetimeSubscription
    extends Subscription {
    ...
}
</programlisting>
                <para>
The same metadata expressed in XML form:
                </para>
<programlisting>
&lt;entity class="org.mag.subcribe.Subscription"&gt;
    &lt;table name="SUB" schema="CNTRCT"/&gt;
    &lt;inheritance strategy="SINGLE_TABLE"/&gt;
    ...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.LifetimeSubscription"&gt;
    ...
&lt;/entity&gt;
</programlisting>
            </example>
            <para>
Single table inheritance is the default strategy. Thus, we could omit the
<literal>@Inheritance</literal> annotation in the example above and get the same
result.
            </para>
            <note>
                <para>
                <indexterm>
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        flat
                    </secondary>
                </indexterm>
                <indexterm>
                    <primary>
                        flat
                    </primary>
                    <seealso>
                        inheritance
                    </seealso>
                </indexterm>
Mapping subclass state to the superclass table is often called <emphasis>flat
</emphasis> inheritance mapping.
                </para>
            </note>
            <section id="jpa_overview_mapping_inher_single_adv">
                <title>
                    Advantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_single_adv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        SINGLE_TABLE strategy
                    </secondary>
                    <tertiary>
                        advantages
                    </tertiary>
                </indexterm>
                <para>
Single table inheritance mapping is the fastest of all inheritance models, since
it never requires a join to retrieve a persistent instance from the database.
Similarly, persisting or updating a persistent instance requires only a single
<literal>INSERT</literal> or <literal>UPDATE</literal> statement. Finally,
relations to any class within a single table inheritance hierarchy are just as
efficient as relations to a base class.
                </para>
            </section>
            <section id="jpa_overview_mapping_inher_single_disadv">
                <title>
                    Disadvantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_single_disadv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        SINGLE_TABLE strategy
                    </secondary>
                    <tertiary>
                        disadvantages
                    </tertiary>
                </indexterm>
                <para>
The larger the inheritance model gets, the "wider" the mapped table gets, in
that for every field in the entire inheritance hierarchy, a column must exist in
the mapped table. This may have undesirable consequence on the database size,
since a wide or deep inheritance hierarchy will result in tables with many
mostly-empty columns.
                </para>
            </section>
        </section>
        <section id="jpa_overview_mapping_inher_joined">
            <title>
                Joined
            </title>
            <indexterm zone="jpa_overview_mapping_inher_joined">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    inheritance
                </secondary>
                <tertiary>
                    JOINED strategy
                </tertiary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_inher_joined">
                <primary>
                    inheritance
                </primary>
                <secondary>
                    JOINED strategy
                </secondary>
            </indexterm>
            <para>
The <literal>InheritanceType.JOINED</literal> strategy uses a different table
for each class in the hierarchy. Each table only includes state declared in its
class. Thus to load a subclass instance, the JPA implementation must
read from the subclass table as well as the table of each ancestor class, up to
the base entity class.
            </para>
            <note>
                <para>
                <indexterm>
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        vertical
                    </secondary>
                </indexterm>
                <indexterm>
                    <primary>
                        vertical
                    </primary>
                    <seealso>
                        inheritance
                    </seealso>
                </indexterm>
Using joined subclass tables is also called <emphasis>vertical</emphasis>
inheritance mapping.
                </para>
            </note>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 256 x 229 (see README) -->
                    <imagedata fileref="img/jpa-inher-joined.png" width="171px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
<classname>PrimaryKeyJoinColumn</classname> annotations tell the JPA
implementation how to join each subclass table record to the corresponding
record in its direct superclass table. In our model, the <literal>LINE_ITEM.ID
</literal> column joins to the <literal>CONTRACT.ID</literal> column. The
<classname>PrimaryKeyJoinColumn</classname> annotation has the following
properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>String name</literal>: The name of the subclass table column. When
there is a single identity field, defaults to that field's column name.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>String referencedColumnName</literal>: The name of the superclass
table column this subclass table column joins to. When there is a single
identity field, defaults to that field's column name.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>String columnDefinition</literal>: This property has the same meaning
as the <literal>columnDefinition</literal> property on the <classname>Column
</classname> annotation, described in
<xref linkend="jpa_overview_mapping_column"/>.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
The XML equivalent is the <literal>primary-key-join-column</literal> element.
Its attributes mirror the annotation properties described above:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>referenced-column-name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>column-definition</literal>
                    </para>
                </listitem>
            </itemizedlist>
            <para>
The example below shows how we use <literal>InheritanceTable.JOINED</literal>
and a primary key join column to map our sample model according to the diagram
above. Note that a primary key join column is not strictly needed, because there
is only one identity column, and the subclass table column has the same name as
the superclass table column. In this situation, the defaults suffice. However,
we include the primary key join column for illustrative purposes.
            </para>
            <example id="jpa_overview_mapping_inher_joinedex">
                <title>
                    Joined Subclass Tables
                </title>
<programlisting>
@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
public class Contract
    extends Document {
    ...
}

public class Subscription {
    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    @PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
    public static class LineItem
        extends Contract {
        ...
    }
}
</programlisting>
                <para>
The same metadata expressed in XML form:
                </para>
<programlisting>
&lt;entity class="org.mag.subcribe.Contract"&gt;
    &lt;table schema="CNTRCT"/&gt;
    &lt;inheritance strategy="JOINED"/&gt;
    ...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
    &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
    &lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
    ...
&lt;/entity&gt;
</programlisting>
            </example>
            <para>
When there are multiple identity columns, you must define multiple <classname>
PrimaryKeyJoinColumn</classname>s using the aptly-named <classname>
PrimaryKeyJoinColumns</classname> annotation. This annotation's value is an
array of <classname> PrimaryKeyJoinColumn</classname> s. We could rewrite
<classname>LineItem</classname>'s mapping as:
            </para>
<programlisting>
@Entity
@Table(name="LINE_ITEM", schema="CNTRCT")
@PrimaryKeyJoinColumns({
    @PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
})
public static class LineItem
    extends Contract {
    ...
}
</programlisting>
            <para>
In XML, simply list as many <literal> primary-key-join-column</literal> elements
as necessary.
            </para>
            <section id="jpa_overview_mapping_inher_joined_adv">
                <title>
                    Advantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_joined_adv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        JOINED strategy
                    </secondary>
                    <tertiary>
                        advantages
                    </tertiary>
                </indexterm>
                <para>
The joined strategy has the following advantages:
                </para>
                <orderedlist>
                    <listitem>
                        <para>
                        <indexterm>
                            <primary>
                                normalized
                            </primary>
                        </indexterm>
Using joined subclass tables results in the most <emphasis>normalized</emphasis>
database schema, meaning the schema with the least spurious or redundant data.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
As more subclasses are added to the data model over time, the only schema
modification that needs to be made is the addition of corresponding subclass
tables in the database (rather than having to change the structure of existing
tables).
                        </para>
                    </listitem>
                    <listitem>
                        <para>
Relations to a base class using this strategy can be loaded through standard
joins and can use standard foreign keys, as opposed to the machinations required
to load polymorphic relations to table-per-class base types, described below.
                        </para>
                    </listitem>
                </orderedlist>
            </section>
            <section id="jpa_overview_mapping_inher_joined_disadv">
                <title>
                    Disadvantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_joined_disadv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        JOINED strategy
                    </secondary>
                    <tertiary>
                        disadvantages
                    </tertiary>
                </indexterm>
                <para>
Aside from certain uses of the table-per-class strategy described below, the
joined strategy is often the slowest of the inheritance models. Retrieving any
subclass requires one or more database joins, and storing subclasses requires
multiple <literal>INSERT</literal> or <literal>UPDATE</literal> statements. This
is only the case when persistence operations are performed on subclasses; if
most operations are performed on the least-derived persistent superclass, then
this mapping is very fast.
                </para>
                <note>
                    <para>
When executing a select against a hierarchy that uses joined subclass table
inheritance, you must consider how to load subclass state.
<xref linkend="ref_guide_perfpack_eager"/> in the Reference Guide
describes OpenJPA's options for efficient data loading.
                    </para>
                </note>
            </section>
        </section>
        <section id="jpa_overview_mapping_inher_tpc">
            <title>
                Table Per Class
            </title>
            <indexterm zone="jpa_overview_mapping_inher_tpc">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    inheritance
                </secondary>
                <tertiary>
                    TABLE_PER_CLASS strategy
                </tertiary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_inher_tpc">
                <primary>
                    inheritance
                </primary>
                <secondary>
                    TABLE_PER_CLASS strategy
                </secondary>
            </indexterm>
            <para>
Like the <literal>JOINED</literal> strategy, the <literal>
InheritanceType.TABLE_PER_CLASS</literal> strategy uses a different table for
each class in the hierarchy. Unlike the <literal>JOINED</literal> strategy,
however, each table includes all state for an instance of the corresponding
class. Thus to load a subclass instance, the JPA implementation must
only read from the subclass table; it does not need to join to superclass
tables.
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 283 x 247 (see README) -->
                    <imagedata fileref="img/inher-tpc.png" width="189px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
Suppose that our sample model's <classname>Magazine</classname> class has a
subclass <classname>Tabloid</classname>. The classes are mapped using the
table-per-class strategy, as in the diagram above. In a table-per-class mapping,
<classname> Magazine</classname>'s table <literal>MAG</literal> contains all
state declared in the base <classname>Magazine</classname> class. <classname>
Tabloid</classname> maps to a separate table, <literal> TABLOID</literal>. This
table contains not only the state declared in the <classname>Tabloid</classname>
subclass, but all the base class state from <classname>Magazine</classname> as
well. Thus the <literal>TABLOID</literal> table would contain columns for
<literal>isbn</literal>, <literal>title</literal>, and other <classname>
Magazine</classname> fields. These columns would default to the names used in
<classname>Magazine</classname>'s mapping metadata.
<xref linkend="jpa_overview_mapping_embed"/> will show you how to use
<literal>AttributeOverride</literal>s and <literal>AssociationOverride</literal>
s to override superclass field mappings.
            </para>
            <example id="jpa_overview_mapping_inher_tpcex">
                <title>
                    Table Per Class Mapping
                </title>
<programlisting>
@Entity
@Table(name="MAG")
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Magazine {
    ...
}

@Entity
@Table(name="TABLOID")
public class Tabloid
    extends Magazine {
    ...
}
</programlisting>
                <para>
And the same classes in XML:
                </para>
<programlisting>
&lt;entity class="org.mag.Magazine"&gt;
    &lt;table name="MAG"/&gt;
    &lt;inheritance strategy="TABLE_PER_CLASS"/&gt;
    ...
&lt;/entity&gt;
&lt;entity class="org.mag.Tabloid"&gt;
    &lt;table name="TABLOID"/&gt;
    ...
&lt;/entity&gt;
</programlisting>
            </example>
            <section id="jpa_overview_mapping_inher_tpc_adv">
                <title>
                    Advantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_tpc_adv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        TABLE_PER_CLASS strategy
                    </secondary>
                    <tertiary>
                        advantages
                    </tertiary>
                </indexterm>
                <para>
The table-per-class strategy is very efficient when operating on instances of a
known class. Under these conditions, the strategy never requires joining to
superclass or subclass tables. Reads, joins, inserts, updates, and deletes are
all efficient in the absence of polymorphic behavior. Also, as in the joined
strategy, adding additional classes to the hierarchy does not require modifying
existing class tables.
                </para>
            </section>
            <section id="jpa_overview_mapping_inher_tpc_disadv">
                <title>
                    Disadvantages
                </title>
                <indexterm zone="jpa_overview_mapping_inher_tpc_disadv">
                    <primary>
                        inheritance
                    </primary>
                    <secondary>
                        TABLE_PER_CLASS strategy
                    </secondary>
                    <tertiary>
                        disadvantages
                    </tertiary>
                </indexterm>
                <para>
Polymorphic relations to non-leaf classes in a table-per-class hierarchy have
many limitations. When the concrete subclass is not known, the related object
could be in any of the subclass tables, making joins through the relation
impossible. This ambiguity also affects identity lookups and queries; these
operations require multiple SQL <literal>SELECT</literal>s (one for each
possible subclass), or a complex <literal>UNION</literal>.
                </para>
                <note>
                    <para>
<xref linkend="ref_guide_mapping_limits_tpc"/> in the Reference Guide
describes the limitations OpenJPA places on table-per-class mapping.
                    </para>
                </note>
            </section>
        </section>
        <section id="jpa_overview_mapping_inher_together">
            <title>
                Putting it All Together
            </title>
            <para>
Now that we have covered JPA's inheritance strategies, we can update our mapping
document with inheritance information. Here is the complete model:
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 513 x 410 (see README) -->
                    <imagedata fileref="img/jpa-inher-all.png" width="341px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
And here is the corresponding mapping metadata:
            </para>
            <example id="jpa_overview_mapping_inher_togetherex">
                <title>
                    Inheritance Mapping
                </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq") 
    private long id;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
    @TableGenerator(name="AuthorGen", table="AUTH_GEN", pkColumnName="PK",
        valueColumnName="AID")
    @Column(name="AID", columnDefinition="INTEGER64")
    private long id;

    ...
}

@Embeddable
public class Address {
    ...
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...
}

@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
public class Contract
    extends Document {
    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Subscription {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    @PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
    public static class LineItem
        extends Contract {
        ...
    }
}

@Entity(name="Lifetime")
public class LifetimeSubscription
    extends Subscription {
    ...
}

@Entity(name="Trial")
public class TrialSubscription
    extends Subscription {
    ...
}
</programlisting>
                <para>
The same metadata expressed in XML form:
                </para>
<programlisting>
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
    version="1.0"&gt;
    &lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/mapped-superclass&gt;
    &lt;entity class="org.mag.Magazine"&gt;
        &lt;table name="MAG"/&gt;
        &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
        &lt;attributes&gt;
            &lt;id name="isbn"&gt;
                &lt;column length="9"/&gt;
            &lt;/id&gt;
            &lt;id name="title"/&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.Article"&gt;
        &lt;table name="ART"&gt;
            &lt;unique-constraint&gt;
               &lt;column-name&gt;TITLE&lt;/column-name&gt;
            &lt;/unique-constraint&gt;
        &lt;/table&gt;
        &lt;sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
            &lt;/id&gt;
           ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Company"&gt;
        &lt;table name="COMP"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="CID"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Author"&gt;
        &lt;table name="AUTH"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="AID" column-definition="INTEGER64"/&gt;
                &lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
                &lt;table-generator name="AuthorGen" table="AUTH_GEN" 
                    pk-column-name="PK" value-column-name="AID"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Contract"&gt;
        &lt;table schema="CNTRCT"/&gt;
        &lt;inheritance strategy="JOINED"/&gt;
        &lt;attributes&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Subscription"&gt;
        &lt;table name="SUB" schema="CNTRCT"/&gt;
        &lt;inheritance strategy="SINGLE_TABLE"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
        &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
        &lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
        ...
    &lt;/entity&gt;
&lt;/entity-mappings&gt;
</programlisting>
            </example>
        </section>
    </section>
    <section id="jpa_overview_mapping_discrim">
        <title>
            Discriminator
        </title>
        <indexterm zone="jpa_overview_mapping_discrim">
            <primary>
                discriminator
            </primary>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_discrim">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                discriminator
            </secondary>
            <seealso>
                discriminator
            </seealso>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_discrim">
            <primary>
                inheritance
            </primary>
            <secondary>
                discriminator
            </secondary>
            <seealso>
                discriminator
            </seealso>
        </indexterm>
        <para>
The <link linkend="jpa_overview_mapping_inher_single">single table</link>
inheritance strategy results in a single table containing records for two or
more different classes in an inheritance hierarchy. Similarly, using the
<link linkend="jpa_overview_mapping_inher_joined"> joined</link> strategy
results in the superclass table holding records for superclass instances as well
as for the superclass state of subclass instances. When selecting data, JPA
needs a way to differentiate a row representing an object of one class from a
row representing an object of another. That is the job of the <emphasis>
discriminator</emphasis> column.
        </para>
        <para>
The discriminator column is always in the table of the base entity. It holds a
different value for records of each class, allowing the JPA runtime
to determine what class of object each row represents.
        </para>
        <para>
The <classname>DiscriminatorColumn</classname> annotation represents a
discriminator column. It has these properties:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>String name</literal>: The column name. Defaults to <literal>DTYPE
</literal>.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>length</literal>: For string discriminator values, the length of the
column. Defaults to 31.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>String columnDefinition</literal>: This property has the same meaning
as the <literal>columnDefinition</literal> property on the <classname>Column
</classname> annotation, described in
<xref linkend="jpa_overview_mapping_column"/>.
                </para>
            </listitem>
            <listitem>
                <para>
<literal>DiscriminatorType discriminatorType</literal>: Enum value declaring
the discriminator strategy of the hierarchy.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The corresponding XML element is <literal> discriminator-column</literal>. Its
attributes mirror the annotation properties above:
        </para>
        <itemizedlist>
            <listitem>
                <para>
<literal>name</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>length</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>column-definition</literal>
                </para>
            </listitem>
            <listitem>
                <para>
<literal>discriminator-type</literal>: One of <literal>STRING</literal>,
<literal>CHAR</literal>, or <literal>INTEGER</literal>.
                </para>
            </listitem>
        </itemizedlist>
        <para>
The <classname>DiscriminatorValue</classname> annotation specifies the
discriminator value for each class. Though this annotation's value is always a
string, the implementation will parse it according to the <classname>
DiscriminatorColumn</classname>'s <literal>discriminatorType</literal> property
above. The type defaults to <literal>DiscriminatorType.STRING</literal>, but
may be <literal> DiscriminatorType.CHAR</literal> or <literal>
DiscriminatorType.INTEGER</literal>. If you do not specify a <classname>
DiscriminatorValue</classname>, the provider will choose an appropriate
default.
        </para>
        <para>
The corresponding XML element is <literal>discriminator-value</literal>. The
text within this element is parsed as the discriminator value.
        </para>
        <note>
            <para>
OpenJPA assumes your model employs a discriminator column if any of the
following are true:
            </para>
            <orderedlist>
                <listitem>
                    <para>
The base entity explicitly declares an inheritance type of <literal>
SINGLE_TABLE</literal>.
                    </para>
                </listitem>
                <listitem>
                    <para>
The base entity sets a discriminator value.
                    </para>
                </listitem>
                <listitem>
                    <para>
The base entity declares a discriminator column.
                    </para>
                </listitem>
            </orderedlist>
            <para>
Only <literal>SINGLE_TABLE</literal> inheritance hierarchies require a
discriminator column and values. <literal> JOINED</literal> hierarchies can use
a discriminator to make some operations more efficient, but do not require one.
<literal>TABLE_PER_CLASS</literal> hierarchies have no use for a discriminator.
            </para>
            <para>
OpenJPA defines additional discriminator strategies; see
<xref linkend="ref_guide_mapping_jpa"/> in the Reference Guide for
details. OpenJPA also supports final entity classes. OpenJPA does not use a
discriminator on final classes.
            </para>
        </note>
        <para>
We can now translate our newfound knowledge of JPA discriminators into concrete
JPA mappings. We first extend our diagram with discriminator columns:
        </para>
        <mediaobject>
            <imageobject>
                <!-- PNG image data, 513 x 410 (see README) -->
                <imagedata fileref="img/jpa-discrim-all.png" width="341px"/>
                
            </imageobject>
        </mediaobject>
        <para>
Next, we present the updated mapping document. Notice that in this version, we
have removed explicit inheritance annotations when the defaults sufficed. Also,
notice that entities using the default <literal>DTYPE</literal> discriminator
column mapping do not need an explicit <classname>DiscriminatorColumn
</classname> annotation.
        </para>
        <example id="jpa_overview_mapping_discrimex">
            <title>
                Discriminator Mapping
            </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
@DiscriminatorValue("Mag")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq") 
    private long id;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
    @TableGenerator(name="AuthorGen", table="AUTH_GEN", pkColumnName="PK",
        valueColumnName="AID")
    @Column(name="AID", columnDefinition="INTEGER64")
    private long id;

    ...
}

@Embeddable
public class Address {
    ...
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...
}

@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
@DiscriminatorColumn(name="CTYPE")
public class Contract
    extends Document {
    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
@DiscriminatorColumn(name="KIND", discriminatorType=DiscriminatorType.INTEGER)
@DiscriminatorValue("1")
public class Subscription {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {
        ...
    }
}

@Entity(name="Lifetime")
@DiscriminatorValue("2")
public class LifetimeSubscription
    extends Subscription {
    ...
}

@Entity(name="Trial")
@DiscriminatorValue("3")
public class TrialSubscription
    extends Subscription {
    ...
}
</programlisting>
            <para>
The same metadata expressed in XML:
            </para>
<programlisting>
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
    version="1.0"&gt;
    &lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/mapped-superclass&gt;
    &lt;entity class="org.mag.Magazine"&gt;
        &lt;table name="MAG"/&gt;
        &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
        &lt;discriminator-value&gt;Mag&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;id name="isbn"&gt;
                &lt;column length="9"/&gt;
            &lt;/id&gt;
            &lt;id name="title"/&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.Article"&gt;
        &lt;table name="ART"&gt;
            &lt;unique-constraint&gt;
                &lt;column-name&gt;TITLE&lt;/column-name&gt;
            &lt;/unique-constraint&gt;
        &lt;/table&gt;
        &lt;sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Company"&gt;
        &lt;table name="COMP"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="CID"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Author"&gt;
        &lt;table name="AUTH"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="AID" column-definition="INTEGER64"/&gt;
                &lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
                &lt;table-generator name="AuthorGen" table="AUTH_GEN" 
                    pk-column-name="PK" value-column-name="AID"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Contract"&gt;
        &lt;table schema="CNTRCT"/&gt;
        &lt;inheritance strategy="JOINED"/&gt;
        &lt;discriminator-column name="CTYPE"/&gt;
        &lt;attributes&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Subscription"&gt;
        &lt;table name="SUB" schema="CNTRCT"/&gt;
        &lt;inheritance strategy="SINGLE_TABLE"/&gt;
        &lt;discriminator-value&gt;1&lt;/discriminator-value&gt;
        &lt;discriminator-column name="KIND" discriminator-type="INTEGER"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
        &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
        &lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
        &lt;discriminator-value&gt;2&lt;/discriminator-value&gt;
        ...
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
        &lt;discriminator-value&gt;3&lt;/discriminator-value&gt;
        ...
    &lt;/entity&gt;
&lt;/entity-mappings&gt;
</programlisting>
        </example>
    </section>
    <section id="jpa_overview_mapping_field">
        <title>
            Field Mapping
        </title>
        <indexterm zone="jpa_overview_mapping_field">
            <primary>
                mapping metadata
            </primary>
            <secondary>
                field mapping
            </secondary>
            <seealso>
                persistent fields
            </seealso>
        </indexterm>
        <indexterm zone="jpa_overview_mapping_field">
            <primary>
                persistent fields
            </primary>
            <secondary>
                mapping metadata
            </secondary>
        </indexterm>
        <para>
The following sections enumerate the myriad of field mappings JPA
supports. JPA augments the persistence metadata covered in
<xref linkend="jpa_overview_meta"/> with many new object-relational
annotations. As we explore the library of standard mappings, we introduce each
of these enhancements in context.
        </para>
        <note>
            <para>
OpenJPA supports many additional field types, and allows you to create custom
mappings for unsupported field types or database schemas. See the Reference
Guide's <xref linkend="ref_guide_mapping"/> for complete coverage of
OpenJPA's mapping capabilities.
            </para>
        </note>
        <section id="jpa_overview_mapping_basic">
            <title>
                Basic Mapping
            </title>
            <indexterm zone="jpa_overview_mapping_basic">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    basic fields
                </secondary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_basic">
                <primary>
                    persistent fields
                </primary>
                <secondary>
                    basic
                </secondary>
            </indexterm>
            <para>
A <emphasis>basic</emphasis> field mapping stores the field value directly into
a database column. The following field metadata types use basic mapping. These
types were defined in <xref linkend="jpa_overview_meta_field"/>.
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<link linkend="jpa_overview_meta_id"><classname>Id</classname></link> fields.
                    </para>
                </listitem>
                <listitem>
                    <para>
<link linkend="jpa_overview_meta_version"><classname> Version</classname></link>
fields.
                    </para>
                </listitem>
                <listitem>
                    <para>
<link linkend="jpa_overview_meta_basic"><classname>Basic</classname></link>
fields.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
In fact, you have already seen examples of basic field mappings in this chapter
- the mapping of all identity fields in
<xref linkend="jpa_overview_mapping_identityex"/>. As you saw in that
section, to write a basic field mapping you use the <classname>Column
</classname> annotation to describe the column the field value is stored in. We
discussed the <classname>Column</classname> annotation in
<xref linkend="jpa_overview_mapping_column"/>. Recall that the name of
the column defaults to the field name, and the type of the column defaults to an
appropriate type for the field type. These defaults allow you to sometimes omit
the annotation altogether.
            </para>
            <section id="jpa_overview_mapping_lob">
                <title>
                    LOBs
                </title>
                <indexterm zone="jpa_overview_mapping_lob">
                    <primary>
                        LOB
                    </primary>
                </indexterm>
                <indexterm zone="jpa_overview_mapping_lob">
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        LOB types
                    </secondary>
                </indexterm>
                <indexterm zone="jpa_overview_mapping_lob">
                    <primary>
                        annotations
                    </primary>
                    <secondary>
                        Lob
                    </secondary>
                </indexterm>
                <para>
Adding the <classname>Lob</classname> marker annotation to a basic field signals
that the data is to be stored as a LOB (Large OBject). If the field holds string
or character data, it will map to a <literal>CLOB</literal> (Character Large
OBject) database column. If the field holds any other data type, it will be
stored as binary data in a <literal>BLOB</literal> (Binary Large OBject) column.
The implementation will serialize the Java value if needed.
                </para>
                <para>
The equivalent XML element is <literal>lob</literal>, which has no children or
attributes.
                </para>
                <note>
                    <para>
OpenJPA also supports LOB streaming. See <xref linkend="ref_guide_streamsupport"/> in
the Reference Guide for details.
                    </para>
                </note>
            </section>
            <section id="jpa_overview_mapping_enum">
                <title>
                    Enumerated
                </title>
                <indexterm zone="jpa_overview_mapping_enum">
                    <primary>
                        Enumerated
                    </primary>
                </indexterm>
                <indexterm zone="jpa_overview_mapping_enum">
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        enums
                    </secondary>
                </indexterm>
                <indexterm zone="jpa_overview_mapping_enum">
                    <primary>
                        annotations
                    </primary>
                    <secondary>
                        Enumerated
                    </secondary>
                </indexterm>
                <para>
You can apply the <classname>Enumerated</classname> annotation to your
<classname>Enum</classname> fields to control how they map to the database. The
<classname>Enumerated</classname> annotation's value one of the following
constants from the <classname>EnumType</classname> enum:
                </para>
                <itemizedlist>
                    <listitem>
                        <para>
<literal>EnumType.ORDINAL</literal>: The default. The persistence
implementation places the ordinal value of the enum in a numeric column. This is
an efficient mapping, but may break if you rearrange the Java enum declaration.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
<literal>EnumType.STRING</literal>: Store the name of the enum value rather
than the ordinal. This mapping uses a <literal>VARCHAR</literal> column rather
than a numeric one.
                        </para>
                    </listitem>
                </itemizedlist>
                <para>
The <classname>Enumerated</classname> annotation is optional. Any un-annotated
enumeration field defaults to <literal>ORDINAL</literal> mapping.
                </para>
                <para>
The corresponding XML element is <literal>enumerated</literal>. Its embedded
text must be one of <literal>STRING</literal> or <literal>ORIDINAL</literal>.
                </para>
            </section>
            <section id="jpa_overview_mapping_temporal">
                <title>
                    Temporal Types
                </title>
                <indexterm zone="jpa_overview_mapping_temporal">
                    <primary>
                        mapping metadata
                    </primary>
                    <secondary>
                        temporal types
                    </secondary>
                </indexterm>
                <indexterm zone="jpa_overview_mapping_temporal">
                    <primary>
                        persistent fields
                    </primary>
                    <secondary>
                        temporal
                    </secondary>
                </indexterm>
                <para>
The <classname>Temporal</classname> annotation determines how the implementation
handles your basic <classname> java.util.Date</classname> and <classname>
java.util.Calendar</classname> fields at the JDBC level. The <classname>
Temporal</classname> annotation's value is a constant from the <classname>
TemporalType</classname> enum. Available values are:
                </para>
                <itemizedlist>
                    <listitem>
                        <para>
<literal>TemporalType.TIMESTAMP</literal>: The default. Use JDBC's timestamp
APIs to manipulate the column data.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
<literal>TemporalType.DATE</literal>: Use JDBC's SQL date APIs to manipulate
the column data.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
<literal>TemporalType.TIME</literal>: Use JDBC's time APIs to manipulate the
column data.
                        </para>
                    </listitem>
                </itemizedlist>
                <para>
If the <classname>Temporal</classname> annotation is omitted, the implementation
will treat the data as a timestamp.
                </para>
                <para>
The corresponding XML element is <literal>temporal</literal>, whose text value
must be one of: <literal>TIME</literal>, <literal>DATE</literal>, or <literal>
TIMESTAMP</literal>.
                </para>
            </section>
            <section id="jpa_overview_mapping_basic_example">
                <title>
                    The Updated Mappings
                </title>
                <para>
Below we present an updated diagram of our model and its associated database
schema, followed by the corresponding mapping metadata. Note that the mapping
metadata relies on defaults where possible. Also note that as a mapped
superclass, <classname>Document</classname> can define mappings that will
automatically transfer to its subclass' tables. In
<xref linkend="jpa_overview_mapping_embed"/>, you will see how a subclass
can override its mapped superclass' mappings.
                </para>
                <mediaobject>
                    <imageobject>
                        <!-- PNG image data, 580 x 553 (see README) -->
                        <imagedata fileref="img/jpa-basic-field.png" width="387px"/>
                        
                    </imageobject>
                </mediaobject>
                <example id="jpa_overview_mapping_basicex">
                    <title>
                        Basic Field Mapping
                    </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
@DiscriminatorValue("Mag")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    @Column(name="VERS")
    @Version private int version;
    
    private String name;
    private double price;

    @Column(name="COPIES")
    private int copiesSold;

    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq") 
    private long id;

    @Column(name="VERS")
    @Version private int version;

    private String title;
    private byte[] content;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    @Column(name="VERS")
    @Version private int version;

    private String name;

    @Column(name="REV")
    private double revenue;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
    @TableGenerator(name="AuthorGen", table="AUTH_GEN", pkColumnName="PK",
        valueColumnName="AID")
    @Column(name="AID", columnDefinition="INTEGER64")
    private long id;

    @Column(name="VERS")
    @Version private int version;

    @Column(name="FNAME")
    private String firstName;

    @Column(name="LNAME")
    private String lastName;

    ...
}

@Embeddable
public class Address {
    ...
}

package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    @Column(name="VERS")
    @Version private int version;

    ...
}

@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
@DiscriminatorColumn(name="CTYPE")
public class Contract
    extends Document {

    @Lob
    private String terms;

    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
@DiscriminatorColumn(name="KIND", discriminatorType=DiscriminatorType.INTEGER)
@DiscriminatorValue("1")
public class Subscription {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    @Column(name="VERS")
    @Version private int version;

    @Column(name="START")
    private Date startDate;

    @Column(name="PAY")
    private double payment;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {

        @Column(name="COMM")
        private String comments;

        private double price;
        private long num;
        ...
    }
}

@Entity(name="Lifetime")
@DiscriminatorValue("2")
public class LifetimeSubscription
    extends Subscription {

    @Basic(fetch=FetchType.LAZY)
    @Column(name="ELITE")
    private boolean getEliteClub() { ... }
    public void setEliteClub(boolean elite) { ... }

    ...
}

@Entity(name="Trial")
@DiscriminatorValue("3")
public class TrialSubscription
    extends Subscription {

    @Column(name="END")
    public Date getEndDate() { ... }
    public void setEndDate(Date end) { ... }

    ...
}
</programlisting>
                    <para>
The same metadata expressed in XML:
                    </para>
<programlisting>
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
    version="1.0"&gt;
    &lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            ...
        &lt;/attributes&gt;
    &lt;/mapped-superclass&gt;
    &lt;entity class="org.mag.Magazine"&gt;
        &lt;table name="MAG"/&gt;
        &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
        &lt;discriminator-value&gt;Mag&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;id name="isbn"&gt;
                &lt;column length="9"/&gt;
            &lt;/id&gt;
            &lt;id name="title"/&gt;
            &lt;basic name="name"/&gt;
            &lt;basic name="price"/&gt;
            &lt;basic name="copiesSold"&gt;
                &lt;column name="COPIES"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.Article"&gt;
        &lt;table name="ART"&gt;
            &lt;unique-constraint&gt;
               &lt;column-name&gt;TITLE&lt;/column-name&gt;
            &lt;/unique-constraint&gt;
        &lt;/table&gt;
        &lt;sequence-generator name="ArticleSeq", sequenceName="ART_SEQ"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
            &lt;/id&gt;
            &lt;basic name="title"/&gt;
            &lt;basic name="content"/&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Company"&gt;
        &lt;table name="COMP"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="CID"/&gt;
            &lt;/id&gt;
            &lt;basic name="name"/&gt;
            &lt;basic name="revenue"&gt;
                &lt;column name="REV"/&gt;
            &lt;/basic&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Author"&gt;
        &lt;table name="AUTH"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="AID" column-definition="INTEGER64"/&gt;
                &lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
                &lt;table-generator name="AuthorGen" table="AUTH_GEN" 
                    pk-column-name="PK" value-column-name="AID"/&gt;
            &lt;/id&gt;
            &lt;basic name="firstName"&gt;
                &lt;column name="FNAME"/&gt;
            &lt;/basic&gt;
            &lt;basic name="lastName"&gt;
                &lt;column name="LNAME"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Contract"&gt;
        &lt;table schema="CNTRCT"/&gt;
        &lt;inheritance strategy="JOINED"/&gt;
        &lt;discriminator-column name="CTYPE"/&gt;
        &lt;attributes&gt;
            &lt;basic name="terms"&gt;
                &lt;lob/&gt;
            &lt;/basic&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Subscription"&gt;
        &lt;table name="SUB" schema="CNTRCT"/&gt;
        &lt;inheritance strategy="SINGLE_TABLE"/&gt;
        &lt;discriminator-value&gt;1&lt;/discriminator-value&gt;
        &lt;discriminator-column name="KIND" discriminator-type="INTEGER"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            &lt;basic name="payment"&gt;
                &lt;column name="PAY"/&gt;
            &lt;/basic&gt;
            &lt;basic name="startDate"&gt;
                &lt;column name="START"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
        &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
        &lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
        &lt;attributes&gt;
            &lt;basic name="comments"&gt;
                &lt;column name="COMM"/&gt;
            &lt;/basic&gt;
            &lt;basic name="price"/&gt;
            &lt;basic name="num"/&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
        &lt;discriminator-value&gt;2&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;basic name="eliteClub" fetch="LAZY"&gt;
                &lt;column name="ELITE"/&gt;
            &lt;/basic&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
        &lt;discriminator-value&gt;3&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;basic name="endDate"&gt;
                &lt;column name="END"/&gt;
            &lt;/basic&gt;
            ...
        &lt;/attributes&gt;
    &lt;/entity&gt;
&lt;/entity-mappings&gt;
</programlisting>
                </example>
            </section>
        </section>
        <section id="jpa_overview_mapping_secondary">
            <title>
                Secondary Tables
            </title>
            <indexterm zone="jpa_overview_mapping_secondary">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    secondary table fields
                </secondary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_secondary">
                <primary>
                    persistent fields
                </primary>
                <secondary>
                    in secondary tables
                </secondary>
            </indexterm>
            <para>
Sometimes a logical record is spread over multiple database tables. JPA
calls a class' declared table the <emphasis>primary</emphasis>
table, and calls other tables that make up a logical record <emphasis>secondary
</emphasis> tables. You can map any persistent field to a secondary table. Just
write the standard field mapping, then perform these two additional steps:
            </para>
            <orderedlist>
                <listitem>
                    <para>
Set the <literal>table</literal> attribute of each of the field's columns or
join columns to the name of the secondary table.
                    </para>
                </listitem>
                <listitem>
                    <para>
Define the secondary table on the entity class declaration.
                    </para>
                </listitem>
            </orderedlist>
            <para>
You define secondary tables with the <classname>SecondaryTable</classname>
annotation. This annotation has all the properties of the <classname>Table
</classname> annotation covered in <xref linkend="jpa_overview_mapping_table"/>
, plus a <literal> pkJoinColumns</literal> property.
            </para>
            <para>
The <literal>pkJoinColumns</literal> property is an array of <classname>
PrimaryKeyJoinColumn</classname>s dictating how to join secondary table records
to their owning primary table records. Each <classname>PrimaryKeyJoinColumn
</classname> joins a secondary table column to a primary key column in the
primary table. See <xref linkend="jpa_overview_mapping_inher_joined"/>
above for coverage of <classname>PrimaryKeyJoinColumn</classname>'s properties.
            </para>
            <para>
The corresponding XML element is <literal>secondary-table</literal>. This
element has all the attributes of the <literal>table</literal> element, but also
accepts nested <literal>primary-key-join-column</literal> elements.
            </para>
            <para>
In the following example, we move the <literal>Article.content</literal> field
we mapped in <xref linkend="jpa_overview_mapping_basic"/> into a joined
secondary table, like so:
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 284 x 167 (see README) -->
                    <imagedata fileref="img/secondary-table.png" width="189px"/>
                    
                </imageobject>
            </mediaobject>
            <example id="jpa_overview_mapping_secondaryex">
                <title>
                    Secondary Table Field Mapping
                </title>
<programlisting>
package org.mag;

@Entity
@Table(name="ART")
@SecondaryTable(name="ART_DATA", 
    pkJoinColumns=@PrimaryKeyJoinColumn(name="ART_ID", referencedColumnName="ID"))
public class Article {

    @Id private long id;

    @Column(table="ART_DATA")
    private byte[] content;

    ...
}
</programlisting>
                <para>
And in XML:
                </para>
<programlisting>
&lt;entity class="org.mag.Article"&gt;
    &lt;table name="ART"/&gt;
    &lt;secondary-table name="ART_DATA"&gt;
        &lt;primary-key-join-column name="ART_ID" referenced-column-name="ID"/&gt;
    &lt;/secondary-table&gt;
    &lt;attributes&gt;
        &lt;id name="id"/&gt;
        &lt;basic name="content"&gt;
            &lt;column table="ART_DATA"/&gt;
        &lt;/basic&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
        </section>
        <section id="jpa_overview_mapping_embed">
            <title>
                Embedded Mapping
            </title>
            <indexterm zone="jpa_overview_mapping_embed">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    embedded fields
                </secondary>
                <seealso>
                    embedded
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_embed">
                <primary>
                    embedded
                </primary>
                <secondary>
                    mapping embedded fields
                </secondary>
            </indexterm>
            <para>
<xref linkend="jpa_overview_meta"/> describes JPA's concept of <emphasis>
embeddable</emphasis> objects. The field values of embedded objects are stored
as part of the owning record, rather than as a separate database record. Thus,
instead of mapping a relation to an embeddable object as a foreign key, you map
all the fields of the embeddable instance to columns in the owning field's
table.
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 464 x 203 (see README) -->
                    <imagedata fileref="img/jpa-embedded.png" width="309px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
JPA defaults the embedded column names and descriptions to those of
the embeddable class' field mappings. The <classname> AttributeOverride
</classname> annotation overrides a basic embedded mapping. This annotation has
the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>String name</literal>: The name of the embedded class' field being
mapped to this class' table.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>Column column</literal>: The column defining the mapping of the
embedded class' field to this class' table.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
The corresponding XML element is <literal> attribute-override</literal>. It has
a single <literal>name</literal> attribute to name the field being overridden,
and a single <literal>column</literal> child element.
            </para>
            <para>
To declare multiple overrides, use the <classname>AttributeOverrides</classname>
annotation, whose value is an array of <classname>AttributeOverride</classname>
s. In XML, simply list multiple <literal>attribute-override</literal> elements
in succession.
            </para>
            <para>
To override a many to one or one to one relationship, use the <classname>
AssociationOverride</classname> annotation in place of <classname>
AttributeOverride</classname>. <classname> AssociationOverride</classname> has
the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>String name</literal>: The name of the embedded class' field being
mapped to this class' table.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>JoinColumn[] joinColumns</literal>: The foreign key columns joining to
the related record.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
The corresponding XML element is <literal> association-override</literal>. It
has a single <literal>name</literal> attribute to name the field being
overridden, and one or more <literal>join-column</literal> child elements.
            </para>
            <para>
To declare multiple relation overrides, use the <classname> AssociationOverrides
</classname> annotation, whose value is an array of <classname>
AssociationOverride</classname> s. In XML, simply list multiple <literal>
association-override</literal> elements in succession.
            </para>
            <example id="jpa_overview_mapping_embedex">
                <title>
                    Embedded Field Mapping
                </title>
                <para>
In this example, <classname>Company</classname> overrides the default mapping of
<literal>Address.street</literal> and <literal>Address.city</literal>. All
other embedded mappings are taken from the <classname>Address</classname>
embeddable class.
                </para>
<programlisting>
package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Embedded
    @AttributeOverrides({
        @AttributeOverride(name="street", column=@Column(name="STRT")),
        @AttributeOverride(name="city", column=@Column(name="ACITY"))
    })
    private Address address;
    
    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    // use all defaults from Address class mappings
    private Address address;
    
    ...
}

@Embeddable
public class Address {

    private String street;
    private String city;
    @Column(columnDefinition="CHAR(2)")
    private String state;
    private String zip;
}
</programlisting>
                <para>
The same metadata expressed in XML:
                </para>
<programlisting>
&lt;entity class="org.mag.pub.Company"&gt;
    &lt;table name="COMP"/&gt;
    &lt;attributes&gt;
        ...
        &lt;embedded name="address"&gt;
            &lt;attribute-override name="street"&gt;
               &lt;column name="STRT"/&gt;
            &lt;/attribute-override&gt;
            &lt;attribute-override name="city"&gt;
               &lt;column name="ACITY"/&gt;
            &lt;/attribute-override&gt;
        &lt;/embedded&gt;
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Author"&gt;
    &lt;table name="AUTH"/&gt;
    &lt;attributes&gt;
        &lt;embedded name="address"&gt;
            &lt;!-- use all defaults from Address --&gt;
        &lt;/embedded&gt;
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;embeddable class="org.mag.pub.Address"&gt;
    &lt;attributes&gt;
        &lt;basic name="street"/&gt;
        &lt;basic name="city"/&gt;
        &lt;basic name="state"&gt;
            &lt;column column-definition="CHAR(2)"/&gt;
        &lt;/basic&gt;
        &lt;basic name="zip"/&gt;
    &lt;/attributes&gt;
&lt;/embeddable&gt;
</programlisting>
            </example>
            <para>
You can also use attribute overrides on an entity class to override mappings
defined by its mapped superclass or table-per-class superclass. The example
below re-maps the <literal>Document.version</literal> field to the <classname>
Contract</classname> table's <literal>CVERSION</literal> column.
            </para>
            <example id="jpa_overview_mapping_joined_overex">
                <title>
                    Mapping Mapped Superclass Field
                </title>
<programlisting>
@MappedSuperclass
public abstract class Document {

    @Column(name="VERS")
    @Version private int version;

    ...
}

@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
@DiscriminatorColumn(name="CTYPE")
@AttributeOverride(name="version", column=@Column(name="CVERSION"))
public class Contract
    extends Document {
    ...
}
</programlisting>
                <para>
The same metadata expressed in XML form:
                </para>
<programlisting>
&lt;mapped-superclass class="org.mag.subcribe.Document"&gt;
    &lt;attributes&gt;
        &lt;version name="version"&gt;
            &lt;column name="VERS"&gt;
        &lt;/version&gt;
        ...
    &lt;/attributes&gt;
&lt;/mapped-superclass&gt;
&lt;entity class="org.mag.subcribe.Contract"&gt;
    &lt;table schema="CNTRCT"/&gt;
    &lt;inheritance strategy="JOINED"/&gt;
    &lt;discriminator-column name="CTYPE"/&gt;
    &lt;attribute-override name="version"&gt;
        &lt;column name="CVERSION"/&gt;
    &lt;/attribute-override&gt;
    &lt;attributes&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
        </section>
        <section id="jpa_overview_mapping_rel">
            <title>
                Direct Relations
            </title>
            <indexterm zone="jpa_overview_mapping_rel">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    direct relation fields
                </secondary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_rel">
                <primary>
                    persistent fields
                </primary>
                <secondary>
                    direct relations
                </secondary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_rel">
                <primary>
                    one-one
                </primary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_rel">
                <primary>
                    many-one
                </primary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <para>
A direct relation is a non-embedded persistent field that holds a reference to
another entity. <link linkend="jpa_overview_meta_manytoone">many to one</link>
and <link linkend="jpa_overview_meta_onetoone">one to one</link> metadata field
types are mapped as direct relations. Our model has three direct relations:
<classname>Magazine</classname>'s <literal>publisher</literal> field is a direct
relation to a <classname>Company</classname>, <classname>Magazine</classname>'s
<literal>coverArticle</literal> field is a direct relation to <classname>
Article</classname>, and the <literal>LineItem.magazine</literal> field is a
direct relation to a <classname>Magazine</classname>. Direct relations are
represented in the database by foreign key columns:
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 374 x 386 (see README) -->
                    <imagedata fileref="img/jpa-direct-relation.png" width="249px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
You typically map a direct relation with <classname>JoinColumn</classname>
annotations describing how the local foreign key columns join to the primary key
columns of the related record. The <classname>JoinColumn</classname> annotation
exposes the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>String name</literal>: The name of the foreign key column. Defaults to
the relation field name, plus an underscore, plus the name of the referenced
primary key column.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>String referencedColumnName</literal>: The name of the primary key
column being joined to. If there is only one identity field in the related
entity class, the join column name defaults to the name of the identity field's
column.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>boolean unique</literal>: Whether this column is guaranteed to hold
unique values for all rows. Defaults to false.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
<classname>JoinColumn</classname> also has the same <literal> nullable</literal>
, <literal>insertable</literal>, <literal> updatable</literal>, <literal>
columnDefinition</literal>, and <literal>table</literal> properties as the
<classname> Column</classname> annotation. See
<xref linkend="jpa_overview_mapping_column"/> for details on these
properties.
            </para>
            <para>
The <literal>join-column</literal> element represents a join column in XML. Its
attributes mirror the above annotation's properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>referenced-column-name</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>unique</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>nullable</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>insertable</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>updatable</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>column-definition</literal>
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>table</literal>
                    </para>
                </listitem>
            </itemizedlist>
            <para>
When there are multiple columns involved in the join, as when a <classname>
LineItem</classname> references a <classname>Magazine</classname> in our model,
the <classname>JoinColumns</classname> annotation allows you to specify an array
of <classname>JoinColumn</classname> values. In XML, simply list multiple
<literal>join-column</literal> elements.
            </para>
            <note>
                <para>
OpenJPA supports many non-standard joins. See
<xref linkend="ref_guide_mapping_notes_nonstdjoins"/> in the Reference
Guide for details.
                </para>
            </note>
            <example id="jpa_overview_mapping_relex">
                <title>
                    Direct Relation Field Mapping
                </title>
<programlisting>
package org.mag;

@Table(name="AUTH")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    @OneToOne
    @JoinColumn(name="COVER_ID" referencedColumnName="ID")
    private Article coverArticle;

    @ManyToOne
    @JoinColumn(name="PUB_ID" referencedColumnName="CID")
    private Company publisher;

    ...
}

@Table(name="ART")
public class Article {

    @Id private long id;

    ...
}


package org.mag.pub;

@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    ...
}


package org.mag.subscribe;

public class Subscription {
    ...

    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {

        @ManyToOne
        @JoinColumns({
            @JoinColumn(name="MAG_ISBN" referencedColumnName="ISBN"),
            @JoinColumn(name="MAG_TITLE" referencedColumnName="TITLE")
        })
        private Magazine magazine;

        ...
    }
}
</programlisting>
                <para>
The same metadata expressed in XML form:
                </para>
<programlisting>
&lt;entity class="org.mag.Magazine"&gt;
    &lt;table name="MAG"/&gt;
    &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
    &lt;attributes&gt;
        &lt;id name="isbn"&gt;
            &lt;column length="9"/&gt;
        &lt;/id&gt;
        &lt;id name="title"/&gt;
        &lt;one-to-one name="coverArticle"&gt;
            &lt;join-column name="COVER_ID" referenced-column-name="ID"/&gt;
        &lt;/one-to-one&gt;
        &lt;many-to-one name="publisher"&gt;
            &lt;join-column name="PUB_IC" referenced-column-name="CID"/&gt;
        &lt;/many-to-one&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.Article"&gt;
    &lt;table name="ART"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"/&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Company"&gt;
    &lt;table name="COMP"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"&gt;
            &lt;column name="CID"/&gt;
        &lt;/id&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
    &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
    &lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
    &lt;attributes&gt;
        &lt;many-to-one name="magazine"&gt;
            &lt;join-column name="MAG_ISBN" referenced-column-name="ISBN"/&gt;
            &lt;join-column name="MAG_TITLE" referenced-column-name="TITLE"/&gt;
        &lt;/many-to-one&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
            <para>
When the entities in a one to one relation join on shared primary key values
rather than separate foreign key columns, use the <classname>
PrimaryKeyJoinColumn(s)</classname> annotation or <literal>
primary-key-join-column</literal> elements in place of <classname>JoinColumn(s)
</classname> / <literal> join-column</literal> elements.
            </para>
        </section>
        <section id="jpa_overview_mapping_assoccoll">
            <title>
                Join Table
            </title>
            <indexterm zone="jpa_overview_mapping_assoccoll">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    association table collection fields
                </secondary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_assoccoll">
                <primary>
                    persistent fields
                </primary>
                <secondary>
                    join table collections
                </secondary>
            </indexterm>
            <indexterm>
                <primary>
                    join table
                </primary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_assoccoll">
                <primary>
                    one-many
                </primary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_assoccoll">
                <primary>
                    many-many
                </primary>
                <seealso>
                    persistent fields
                </seealso>
            </indexterm>
            <para>
A <emphasis>join table</emphasis> consists of two foreign keys. Each row of a
join table associates two objects together. JPA uses join tables to
represent collections of entity objects: one foreign key refers back to the
collection's owner, and the other refers to a collection element.
            </para>
            <para>
<link linkend="jpa_overview_meta_onetomany">one to many</link> and
<link linkend="jpa_overview_meta_manytomany">many to many</link> metadata field
types can map to join tables. Several fields in our model use join table
mappings, including <literal>Magazine.articles</literal> and <literal>
Article.authors</literal>.
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 391 x 407 (see README) -->
                    <imagedata fileref="img/jpa-assoc-table.png" width="261px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
You define join tables with the <classname>JoinTable</classname> annotation.
This annotation has the following properties:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>String name</literal>: Table name. If not given, the name of the table
defaults to the name of the owning entity's table, plus an underscore, plus the
name of the related entity's table.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>String catalog</literal>: Table catalog.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>String schema</literal>: Table schema.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>JoinColumn[] joinColumns</literal>: Array of <classname>JoinColumn
</classname> showing how to associate join table records with the owning row in
the primary table. This property mirrors the <literal>pkJoinColumns</literal>
property of the <classname> SecondaryTable</classname> annotation in
functionality. See <xref linkend="jpa_overview_mapping_secondary"/> to
refresh your memory on secondary tables.
                    </para>
                    <para>
If this is a bidirectional relation (see
<xref linkend="jpa_overview_meta_mappedby"/> ), the name of a join column
defaults to the inverse field name, plus an underscore, plus the referenced
primary key column name. Otherwise, the join column name defaults to the field's
owning entity name, plus an underscore, plus the referenced primary key column
name.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>JoinColumn[] inverseJoinColumns</literal>: Array of <classname>
JoinColumns</classname> showing how to associate join table records with the
records that form the elements of the collection. These join columns are used
just like the join columns for direct relations, and they have the same naming
defaults. Read <xref linkend="jpa_overview_mapping_rel"/> for a review of
direct relation mapping.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
<literal>join-table</literal> is the corresponding XML element. It has the same
attributes as the <literal>table</literal> element, but includes the ability to
nest <literal>join-column</literal> and <literal>inverse-join-column</literal>
elements as children. We have seen <literal>join-column</literal> elements
already; <literal>inverse-join-column</literal> elements have the same
attributes.
            </para>
            <para>
Here are the join table mappings for the diagram above.
            </para>
            <example id="jpa_overview_mapping_assoccollex">
                <title>
                    Join Table Mapping
                </title>
<programlisting>
package org.mag;

@Entity
@Table(name="MAG")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    @OneToMany(...)
    @OrderBy
    @JoinTable(name="MAG_ARTS",
        joinColumns={
            @JoinColumn(name="MAG_ISBN", referencedColumnName="ISBN"),
            @JoinColumn(name="MAG_TITLE", referencedColumnName="TITLE")
        },
        inverseJoinColumns=@JoinColumn(name="ART_ID", referencedColumnName="ID"))
    private Collection&lt;Article&gt; articles;

    ...
}

@Entity
@Table(name="ART")
public class Article {

    @Id private long id;

    @ManyToMany(cascade=CascadeType.PERSIST)
    @OrderBy("lastName, firstName")
    @JoinTable(name="ART_AUTHS",
        joinColumns=@JoinColumn(name="ART_ID", referencedColumnName="ID"),
        inverseJoinColumns=@JoinColumn(name="AUTH_ID", referencedColumnName="AID"))
    private Collection&lt;Author&gt; authors;

    ...
}


package org.mag.pub;

@Entity
@Table(name="AUTH")
public class Author {

    @Column(name="AID", columnDefinition="INTEGER64")
    @Id private long id;

    ...
}
</programlisting>
                <para>
The same metadata expressed in XML:
                </para>
<programlisting>
&lt;entity class="org.mag.Magazine"&gt;
    &lt;table name="MAG"/&gt;
    &lt;attributes&gt;
        &lt;id name="isbn"&gt;
            &lt;column length="9"/&gt;
        &lt;/id&gt;
        &lt;id name="title"/&gt;
        &lt;one-to-many name="articles"&gt;
            &lt;order-by/&gt;
            &lt;join-table name="MAG_ARTS"&gt;
                &lt;join-column name="MAG_ISBN" referenced-column-name="ISBN"/&gt;
                &lt;join-column name="MAG_TITLE" referenced-column-name="TITLE"/&gt;
                &lt;inverse-join-column name="ART_ID" referenced-column-name="ID"/&gt;
            &lt;/join-table&gt;
        &lt;/one-to-many&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.Article"&gt;
    &lt;table name="ART"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"/&gt;
            &lt;many-to-many name="authors"&gt;
                &lt;order-by&gt;lastName, firstName&lt;/order-by&gt;
                &lt;join-table name="ART_AUTHS"&gt;
                    &lt;join-column name="ART_ID" referenced-column-name="ID"/&gt;
                    &lt;inverse-join-column name="AUTH_ID" referenced-column-name="AID"/&gt;
                &lt;/join-table&gt;
            &lt;cascade&gt;
                &lt;cascade-persist/&gt;
            &lt;/cascade&gt;
            &lt;/many-to-many&gt;
            ...
       &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Author"&gt;
    &lt;table name="AUTH"/&gt;
    &lt;attributes&gt;
        &lt;id name="id"&gt;
            &lt;column name="AID" column-definition="INTEGER64"/&gt;
        &lt;/id&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
        </section>
        <section id="jpa_overview_mapping_bidi">
            <title>
                Bidirectional Mapping
            </title>
            <indexterm zone="jpa_overview_mapping_bidi">
                <primary>
                    bidirectional relations
                </primary>
                <secondary>
                    mapping
                </secondary>
            </indexterm>
            <para>
<xref linkend="jpa_overview_meta_mappedby"/> introduced bidirectional
relations. To map a bidirectional relation, you map one field normally using the
annotations we have covered throughout this chapter. Then you use the <literal>
mappedBy</literal> property of the other field's metadata annotation or the
corresponding <literal>mapped-by</literal> XML attribute to refer to the mapped
field. Look for this pattern in these bidirectional relations as you peruse the
complete mappings below:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
<literal>Magazine.publisher</literal> and <literal>Company.mags</literal>.
                    </para>
                </listitem>
                <listitem>
                    <para>
<literal>Article.authors</literal> and <literal>Author.articles</literal>.
                    </para>
                </listitem>
            </itemizedlist>
        </section>
        <section id="jpa_overview_mapping_map">
            <title>
                Map Mapping
            </title>
            <indexterm zone="jpa_overview_mapping_map">
                <primary>
                    mapping metadata
                </primary>
                <secondary>
                    map fields
                </secondary>
            </indexterm>
            <indexterm zone="jpa_overview_mapping_map">
                <primary>
                    persistent fields
                </primary>
                <secondary>
                    maps
                </secondary>
            </indexterm>
            <para>
All map fields in JPA are modeled on either one to many or many to
many associations. The map key is always derived from an associated entity's
field. Thus map fields use the same mappings as any one to many or many to many
fields, namely dedicated <link linkend="jpa_overview_mapping_assoccoll">join
tables</link> or <link linkend="jpa_overview_mapping_bidi">bidirectional
relations</link>. The only additions are the <classname>MapKey</classname>
annotation and <literal>map-key</literal> element to declare the key field. We
covered these additions in <xref linkend="jpa_overview_meta_mapkey"/>.
            </para>
            <mediaobject>
                <imageobject>
                    <!-- PNG image data, 382 x 274 (see README) -->
                    <imagedata fileref="img/jpa-map.png" width="255px"/>
                    
                </imageobject>
            </mediaobject>
            <para>
The example below maps <classname>Subscription</classname>'s map of <classname>
LineItem</classname>s to the <literal>SUB_ITEMS</literal> join table. The key
for each map entry is the <classname> LineItem</classname>'s <literal>num
</literal> field value.
            </para>
            <example id="jpa_overview_mapping_mapex">
                <title>
                    Join Table Map Mapping
                </title>
<programlisting>
package org.mag.subscribe;

@Entity
@Table(name="SUB", schema="CNTRCT")
public class Subscription {

    @OneToMany(cascade={CascadeType.PERSIST,CascadeType.REMOVE})
    @MapKey(name="num")
    @JoinTable(name="SUB_ITEMS", schema="CNTRCT",
        joinColumns=@JoinColumn(name="SUB_ID"),
        inverseJoinColumns=@JoinColumn(name="ITEM_ID"))
    private Map&lt;Long,LineItem&gt; items;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {

        private long num;

        ...
    }
}
</programlisting>
                <para>
The same metadata expressed in XML:
                </para>
<programlisting>
&lt;entity class="org.mag.subscribe.Subscription"&gt;
    &lt;table name="SUB" schema="CNTRCT"/&gt;
    &lt;attributes&gt;
        ...
        &lt;one-to-many name="items"&gt;
            &lt;map-key name="num"&gt;
            &lt;join-table name="SUB_ITEMS" schema="CNTRCT"&gt;
                &lt;join-column name="SUB_ID"/&gt;
                &lt;inverse-join-column name="ITEM_ID"/&gt;
            &lt;/join-table&gt;
            &lt;cascade&gt;
                &lt;cascade-persist/&gt;
                &lt;cascade-remove/&gt;
            &lt;/cascade&gt;
        &lt;/one-to-many&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
    &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
    &lt;attributes&gt;
        ...
        &lt;basic name="num"/&gt;
        ...
    &lt;/attributes&gt;
&lt;/entity&gt;
</programlisting>
            </example>
        </section>
    </section>
    <section id="jpa_overview_mapping_full">
        <title>
            The Complete Mappings
        </title>
        <para>
We began this chapter with the goal of mapping the following object model:
        </para>
        <mediaobject>
            <imageobject>
                <!-- PNG image data, 553 x 580 (see README) -->
                <imagedata fileref="img/jpa-meta-model.png" width="369px"/>
                
            </imageobject>
        </mediaobject>
        <para>
That goal has now been met. In the course of explaining JPA's object-relational
mapping metadata, we slowly built the requisite schema and mappings for the
complete model. First, the database schema:
        </para>
        <mediaobject>
            <imageobject>
                <!-- PNG image data, 490 x 662 (see README) -->
                <imagedata fileref="img/jpa-data-model.png" width="326px"/>
                
            </imageobject>
        </mediaobject>
        <para>
And finally, the complete entity mappings. We have trimmed the mappings to take
advantage of JPA defaults where possible.
        </para>
        <example id="jpa_overview_mapping_fullex">
            <title>
                Full Entity Mappings
            </title>
<programlisting>
package org.mag;

@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
@DiscriminatorValue("Mag")
public class Magazine {

    @Column(length=9)
    @Id private String isbn;
    @Id private String title;

    @Column(name="VERS")
    @Version private int version;
    
    private String name;
    private double price;

    @Column(name="COPIES")
    private int copiesSold;

    @OneToOne(fetch=FetchType.LAZY, 
        cascade={CascadeType.PERSIST,CascadeType.REMOVE})
    @JoinColumn(name="COVER_ID")
    private Article coverArticle;

    @OneToMany(cascade={CascadeType.PERSIST,CascadeType.REMOVE})
    @OrderBy
    @JoinTable(name="MAG_ARTS",
        joinColumns={
            @JoinColumn(name="MAG_ISBN", referencedColumnName="ISBN"),
            @JoinColumn(name="MAG_TITLE", referencedColumnName="TITLE")
        },
        inverseJoinColumns=@JoinColumn(name="ART_ID"))
    private Collection&lt;Article&gt; articles;

    @ManyToOne(fetch=FetchType.LAZY, cascade=CascadeType.PERSIST)
    @JoinColumn(name="PUB_ID")
    private Company publisher; 

    @Transient private byte[] data;


    ...

    public static class MagazineId {
        ...
    }
}

@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {

    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq") 
    private long id;

    @Column(name="VERS")
    @Version private int version;

    private String title;
    private byte[] content;

    @ManyToMany(cascade=CascadeType.PERSIST)
    @OrderBy("lastName, firstName")
    @JoinTable(name="ART_AUTHS",
        joinColumns=@JoinColumn(name="ART_ID"),
        inverseJoinColumns=@JoinColumn(name="AUTH_ID"))
    private Collection&lt;Author&gt; authors;

    ...
}


package org.mag.pub;

@Entity
@Table(name="COMP")
public class Company {

    @Column(name="CID")
    @Id private long id;

    @Column(name="VERS")
    @Version private int version;

    private String name;

    @Column(name="REV")
    private double revenue;

    @Embedded
    @AttributeOverrides({
        @AttributeOverride(name="street", column=@Column(name="STRT")),
        @AttributeOverride(name="city", column=@Column(name="ACITY"))
    })
    private Address address;
    
    @OneToMany(mappedBy="publisher", cascade=CascadeType.PERSIST)
    private Collection&lt;Magazine&gt; mags;

    @OneToMany(cascade=CascadeType.PERSIST,CascadeType.REMOVE)
    @JoinTable(name="COMP_SUBS",
        joinColumns=@JoinColumn(name="COMP_ID"),
        inverseJoinColumns=@JoinColumn(name="SUB_ID"))
    private Collection&lt;Subscription&gt; subscriptions;

    ...
}

@Entity
@Table(name="AUTH")
public class Author {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
    @TableGenerator(name="AuthorGen", tableName="AUTH_GEN", pkColumnName="PK",
        valueColumnName="AID")
    @Column(name="AID", columnDefinition="INTEGER64")
    private long id;

    @Column(name="VERS")
    @Version private int version;

    @Column(name="FNAME")
    private String firstName;

    @Column(name="LNAME")
    private String lastName;

    private Address address;
    
    @ManyToMany(mappedBy="authors", cascade=CascadeType.PERSIST)
    private Collection&lt;Article&gt; arts;

    ...
}

@Embeddable
public class Address {

    private String street;
    private String city;
    @Column(columnDefinition="CHAR(2)")
    private String state;
    private String zip;
}


package org.mag.subscribe;

@MappedSuperclass
public abstract class Document {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    @Column(name="VERS")
    @Version private int version;

    ...
}

@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
@DiscriminatorColumn(name="CTYPE")
public class Contract
    extends Document {

    @Lob
    private String terms;

    ...
}

@Entity
@Table(name="SUB", schema="CNTRCT")
@DiscriminatorColumn(name="KIND", discriminatorType=DiscriminatorType.INTEGER)
@DiscriminatorValue("1")
public class Subscription {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    @Column(name="VERS")
    @Version private int version;

    @Column(name="START")
    private Date startDate;

    @Column(name="PAY")
    private double payment;

    @OneToMany(cascade={CascadeType.PERSIST,CascadeType.REMOVE})
    @MapKey(name="num")
    @JoinTable(name="SUB_ITEMS", schema="CNTRCT",
        joinColumns=@JoinColumn(name="SUB_ID"),
        inverseJoinColumns=@JoinColumn(name="ITEM_ID"))
    private Map&lt;Long,LineItem&gt; items;

    ...

    @Entity
    @Table(name="LINE_ITEM", schema="CNTRCT")
    public static class LineItem
        extends Contract {

        @Column(name="COMM")
        private String comments;

        private double price;
        private long num;

        @ManyToOne
        @JoinColumns({
             @JoinColumn(name="MAG_ISBN", referencedColumnName="ISBN"),
             @JoinColumn(name="MAG_TITLE", referencedColumnName="TITLE")
        })
        private Magazine magazine;
        ...
    }
}

@Entity(name="Lifetime")
@DiscriminatorValue("2")
public class LifetimeSubscription
    extends Subscription {

    @Basic(fetch=FetchType.LAZY)
    @Column(name="ELITE")
    private boolean getEliteClub() { ... }
    public void setEliteClub(boolean elite) { ... }

    ...
}

@Entity(name="Trial")
@DiscriminatorValue("3")
public class TrialSubscription
    extends Subscription {

    @Column(name="END")
    public Date getEndDate() { ... }
    public void setEndDate(Date end) { ... }

    ...
}
</programlisting>
            <para>
The same metadata expressed in XML form:
            </para>
<programlisting>
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
    version="1.0"&gt;
    &lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
        &lt;/attributes&gt;
    &lt;/mapped-superclass&gt;
    &lt;entity class="org.mag.Magazine"&gt;
        &lt;table name="MAG"/&gt;
        &lt;id-class="org.mag.Magazine.MagazineId"/&gt;
        &lt;discriminator-value&gt;Mag&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;id name="isbn"&gt;
                &lt;column length="9"/&gt;
            &lt;/id&gt;
            &lt;id name="title"/&gt;
            &lt;basic name="name"/&gt;
            &lt;basic name="price"/&gt;
            &lt;basic name="copiesSold"&gt;
                &lt;column name="COPIES"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            &lt;many-to-one name="publisher" fetch="LAZY"&gt;
                &lt;join-column name="PUB_ID"/&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                &lt;/cascade&gt;
            &lt;/many-to-one&gt;
            &lt;one-to-many name="articles"&gt;
                &lt;order-by/&gt;
                &lt;join-table name="MAG_ARTS"&gt;
                    &lt;join-column name="MAG_ISBN" referenced-column-name="ISBN"/&gt;
                    &lt;join-column name="MAG_TITLE" referenced-column-name="TITLE"/&gt;
                    &lt;inverse-join-column name="ART_ID"/&gt;
                &lt;/join-table&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                    &lt;cascade-remove/&gt;
                &lt;/cascade&gt;
            &lt;/one-to-many&gt;
            &lt;one-to-one name="coverArticle" fetch="LAZY"&gt;
                &lt;join-column name="COVER_ID"/&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                    &lt;cascade-remove/&gt;
                &lt;/cascade&gt;
            &lt;/one-to-one&gt;
            &lt;transient name="data"/&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.Article"&gt;
        &lt;table name="ART"&gt;
            &lt;unique-constraint&gt;
               &lt;column-name&gt;TITLE&lt;/column-name&gt;
            &lt;/unique-constraint&gt;
        &lt;/table&gt;
        &lt;sequence-generator name="ArticleSeq", sequenceName="ART_SEQ"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
            &lt;/id&gt;
            &lt;basic name="title"/&gt;
            &lt;basic name="content"/&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            &lt;many-to-many name="articles"&gt;
                &lt;order-by&gt;lastName, firstName&lt;/order-by&gt;
                &lt;join-table name="ART_AUTHS"&gt;
                    &lt;join-column name="ART_ID" referenced-column-name="ID"/&gt;
                    &lt;inverse-join-column name="AUTH_ID" referenced-column-name="AID"/&gt;
                &lt;/join-table&gt;
            &lt;/many-to-many&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Company"&gt;
        &lt;table name="COMP"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="CID"/&gt;
            &lt;/id&gt;
            &lt;basic name="name"/&gt;
            &lt;basic name="revenue"&gt;
                &lt;column name="REV"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            &lt;one-to-many name="mags" mapped-by="publisher"&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                &lt;/cascade&gt;
            &lt;/one-to-many&gt;
            &lt;one-to-many name="subscriptions"&gt;
                &lt;join-table name="COMP_SUBS"&gt;
                    &lt;join-column name="COMP_ID"/&gt;
                    &lt;inverse-join-column name="SUB_ID"/&gt;
                &lt;/join-table&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                    &lt;cascade-remove/&gt;
                &lt;/cascade&gt;
            &lt;/one-to-many&gt;
            &lt;embedded name="address"&gt;
                &lt;attribute-override name="street"&gt;
                   &lt;column name="STRT"/&gt;
                &lt;/attribute-override&gt;
                &lt;attribute-override name="city"&gt;
                   &lt;column name="ACITY"/&gt;
                &lt;/attribute-override&gt;
            &lt;/embedded&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.pub.Author"&gt;
        &lt;table name="AUTH"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;column name="AID" column-definition="INTEGER64"/&gt;
                &lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
                &lt;table-generator name="AuthorGen" table="AUTH_GEN" 
                    pk-column-name="PK" value-column-name="AID"/&gt;
            &lt;/id&gt;
            &lt;basic name="firstName"&gt;
                &lt;column name="FNAME"/&gt;
            &lt;/basic&gt;
            &lt;basic name="lastName"&gt;
                &lt;column name="LNAME"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            &lt;many-to-many name="arts" mapped-by="authors"&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                &lt;/cascade&gt;
            &lt;/many-to-many&gt;
            &lt;embedded name="address"/&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Contract"&gt;
        &lt;table schema="CNTRCT"/&gt;
        &lt;inheritance strategy="JOINED"/&gt;
        &lt;discriminator-column name="CTYPE"/&gt;
        &lt;attributes&gt;
            &lt;basic name="terms"&gt;
                &lt;lob/&gt;
            &lt;/basic&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subcribe.Subscription"&gt;
        &lt;table name="SUB" schema="CNTRCT"/&gt;
        &lt;inheritance strategy="SINGLE_TABLE"/&gt;
        &lt;discriminator-value&gt;1&lt;/discriminator-value&gt;
        &lt;discriminator-column name="KIND" discriminator-type="INTEGER"/&gt;
        &lt;attributes&gt;
            &lt;id name="id"&gt;
                &lt;generated-value strategy="IDENTITY"/&gt;
            &lt;/id&gt;
            &lt;basic name="payment"&gt;
                &lt;column name="PAY"/&gt;
            &lt;/basic&gt;
            &lt;basic name="startDate"&gt;
                &lt;column name="START"/&gt;
            &lt;/basic&gt;
            &lt;version name="version"&gt;
                &lt;column name="VERS"/&gt;
            &lt;/version&gt;
            &lt;one-to-many name="items"&gt;
                &lt;map-key name="num"&gt;
                &lt;join-table name="SUB_ITEMS" schema="CNTRCT"&gt;
                    &lt;join-column name="SUB_ID"/&gt;
                    &lt;inverse-join-column name="ITEM_ID"/&gt;
                &lt;/join-table&gt;
                &lt;cascade&gt;
                    &lt;cascade-persist/&gt;
                    &lt;cascade-remove/&gt;
                &lt;/cascade&gt;
            &lt;/one-to-many&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
        &lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
        &lt;attributes&gt;
            &lt;basic name="comments"&gt;
                &lt;column name="COMM"/&gt;
            &lt;/basic&gt;
            &lt;basic name="price"/&gt;
            &lt;basic name="num"/&gt;
            &lt;many-to-one name="magazine"&gt;
                &lt;join-column name="MAG_ISBN" referenced-column-name="ISBN"/&gt;
                &lt;join-column name="MAG_TITLE" referenced-column-name="TITLE"/&gt;
            &lt;/many-to-one&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
        &lt;discriminator-value&gt;2&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;basic name="eliteClub" fetch="LAZY"&gt;
                &lt;column name="ELITE"/&gt;
            &lt;/basic&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
        &lt;discriminator-value&gt;3&lt;/discriminator-value&gt;
        &lt;attributes&gt;
            &lt;basic name="endDate"&gt;
                &lt;column name="END"/&gt;
            &lt;/basic&gt;
        &lt;/attributes&gt;
    &lt;/entity&gt;
    &lt;embeddable class="org.mag.pub.Address"&gt;
        &lt;attributes&gt;
            &lt;basic name="street"/&gt;
            &lt;basic name="city"/&gt;
            &lt;basic name="state"&gt;
                &lt;column column-definition="CHAR(2)"/&gt;
            &lt;/basic&gt;
            &lt;basic name="zip"/&gt;
        &lt;/attributes&gt;
    &lt;/embeddable&gt;
&lt;/entity-mappings&gt;
</programlisting>
        </example>
    </section>
</chapter>