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| <title>6. Non-Standard Joins</title><link rel="stylesheet" href="css/docbook.css" type="text/css"><base href="display"><meta name="generator" content="DocBook XSL Stylesheets V1.72.0"><link rel="start" href="manual.html" title="Apache OpenJPA 2.1 User's Guide"><link rel="up" href="ref_guide_mapping.html" title="Chapter 7. Mapping"><link rel="prev" href="ref_guide_mapping_factory.html" title="5. Mapping Factory"><link rel="next" href="ref_guide_mapping_jpa.html" title="7. Additional JPA Mappings"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">6. |
| Non-Standard Joins |
| </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ref_guide_mapping_factory.html">Prev</a> </td><th width="60%" align="center">Chapter 7. |
| Mapping |
| </th><td width="20%" align="right"> <a accesskey="n" href="ref_guide_mapping_jpa.html">Next</a></td></tr></table><hr></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_guide_mapping_notes_nonstdjoins"></a>6. |
| Non-Standard Joins |
| </h2></div></div></div><a class="indexterm" name="d0e29519"></a><p> |
| The JPA Overview's <a href="jpa_overview_mapping.html" title="Chapter 13. Mapping Metadata">Chapter 13, <i xmlns:xlink="http://www.w3.org/1999/xlink"> |
| Mapping Metadata |
| </i></a> explains join |
| mapping. All of the examples in that document, however, use "standard" joins, in |
| that there is one foreign key column for each primary key column in the target |
| table. OpenJPA supports additional join patterns, including partial primary key |
| joins, non-primary key joins, and joins using constant values. |
| </p><p> |
| <a class="indexterm" name="d0e29530"></a> |
| In a partial primary key join, the source table only has foreign key columns for |
| a subset of the primary key columns in the target table. So long as this subset |
| of columns correctly identifies the proper row(s) in the referenced table, |
| OpenJPA will function properly. There is no special syntax for expressing a |
| partial primary key join - just do not include column definitions for missing |
| foreign key columns. |
| </p><p> |
| <a class="indexterm" name="d0e29538"></a> |
| In a non-primary key join, at least one of the target columns is not a primary |
| key. Once again, OpenJPA supports this join type with the same syntax as a |
| primary key join. There is one restriction, however: each non-primary key column |
| you are joining to must be controlled by a field mapping that implements the |
| <a xmlns:xlink="http://www.w3.org/1999/xlink" href="../javadoc/org/apache/openjpa/jdbc/meta/Joinable.html" target="_top"><code class="classname"> |
| org.apache.openjpa.jdbc.meta.Joinable</code></a> interface. All built |
| in basic mappings implement this interface, including basic fields of embedded |
| objects. OpenJPA will also respect any custom mappings that implement this |
| interface. See <a href="ref_guide_mapping_custom.html" title="10. Custom Mappings">Section 10, “ |
| Custom Mappings |
| ”</a> for an |
| examination of custom mappings. |
| </p><p> |
| <a class="indexterm" name="d0e29552"></a> |
| Not all joins consist of only links between columns. In some cases you might |
| have a schema in which one of the join criteria is that a column in the source |
| or target table must have some constant value. OpenJPA calls joins involving |
| constant values <span class="emphasis"><em>constant joins</em></span>. |
| </p><p> |
| To form a constant join in JPA mapping, first set the <code class="literal">JoinColumn |
| </code>'s <code class="literal">name</code> attribute to the name of the column. If the |
| column with the constant value is the target of the join, give its fully |
| qualified name in the form <code class="literal"><table name>.<column name> |
| </code>. Next, set the <code class="literal">referencedColumnName</code> attribute to |
| the constant value. If the constant value is a string, place it in single quotes |
| to differentiate it from a column name. |
| </p><div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" cellspacing="0" cellpadding="0" width="285"><tr><td><img src="img/joins-constant.png"></td></tr></table></div><p> |
| Consider the tables above. First, we want to join row <code class="literal">T1.R1</code> |
| to row <code class="literal">T2.R1</code>. If we just join column <code class="literal">T1.FK</code> |
| to <code class="literal">T2.PK1</code>, we will wind up matching both <code class="literal">T2.R1 |
| </code> and <code class="literal"> T2.R2</code>. So in addition to joining <code class="literal"> |
| T1.FK</code> to <code class="literal">T2.PK1</code>, we also have to specify that |
| <code class="literal">T2.PK2</code> has the value <code class="literal">a</code>. Here is how we'd |
| accomplish this in mapping metadata. |
| </p><pre class="programlisting"> |
| @Entity |
| @Table(name="T1") |
| public class ... { |
| |
| @ManyToOne |
| @JoinColumns({ |
| @JoinColumn(name="FK" referencedColumnName="PK1"), |
| @JoinColumn(name="T2.PK2" referencedColumnName="'a'") |
| }); |
| private ...; |
| } |
| </pre><p> |
| Notice that we had to fully qualify the name of column <code class="literal">PK2</code> |
| because it is in the target table. Also notice that we put single quotes around |
| the constant value so that it won't be confused with a column name. You do not |
| need single quotes for numeric constants. For example, the syntax to join |
| <code class="literal">T1.R2</code> to <code class="literal">T2.R4</code> is: |
| </p><pre class="programlisting"> |
| @Entity |
| @Table(name="T1") |
| public class ... { |
| |
| @ManyToOne |
| @JoinColumns({ |
| @JoinColumn(name="FK" referencedColumnName="PK2"), |
| @JoinColumn(name="T2.PK1" referencedColumnName="2") |
| }); |
| private ...; |
| } |
| </pre><p> |
| Finally, from the inverse direction, these joins would look like this: |
| </p><pre class="programlisting"> |
| @Entity |
| @Table(name="T2") |
| public class ... { |
| |
| @ManyToOne |
| @JoinColumns({ |
| @JoinColumn(name="T1.FK" referencedColumnName="PK1"), |
| @JoinColumn(name="PK2" referencedColumnName="'a'") |
| }); |
| private ...; |
| |
| @ManyToOne |
| @JoinColumns({ |
| @JoinColumn(name="T1.FK" referencedColumnName="PK2"), |
| @JoinColumn(name="PK1" referencedColumnName="2") |
| }); |
| private ...; |
| } |
| </pre></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ref_guide_mapping_factory.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ref_guide_mapping.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ref_guide_mapping_jpa.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5. |
| Mapping Factory |
| </td><td width="20%" align="center"><a accesskey="h" href="manual.html">Home</a></td><td width="40%" align="right" valign="top"> 7. |
| Additional JPA Mappings |
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