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| Unique Constraints |
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| Unique Constraints |
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| <p> |
| 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 <code class="classname"> UniqueConstraint</code> |
| 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 <code class="literal">INSERT</code>, <code class="literal">UPDATE |
| </code>, and <code class="literal">DELETE</code> statements. For example, suppose there |
| is a unique constraint on the columns of field <code class="literal">F</code>. In the |
| same transaction, you remove an object <code class="literal">A</code> and persist a new |
| object <code class="literal">B</code>, both with the same <code class="literal">F</code> value. The |
| JPA runtime must ensure that the SQL deleting <code class="literal">A</code> |
| is sent to the database before the SQL inserting <code class="literal">B</code> to avoid a |
| unique constraint violation. |
| </p> |
| <p> |
| <code class="classname">UniqueConstraint</code> has these properties: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| <p> |
| <code class="literal">String name</code>: The name of the constraint. OpenJPA will choose |
| a name if you do not provide one, or will create an anonymous constraint. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">String[] columnNames</code>: The names of the columns the |
| constraint spans. |
| </p> |
| </li></ul></div> |
| <p> |
| In XML, unique constraints are represented by nesting <code class="literal"> |
| unique-constraint</code> elements within the <code class="literal"> table</code> |
| element. Each <code class="literal">unique-constraint</code> element in turn nests |
| <code class="literal">column-name</code> text elements to enumerate the constraint's |
| columns. |
| </p> |
| <div class="example"><a name="jpa_overview_mapping_unq_attrex"></a><p class="title"><b>Example 13.2. |
| Defining a Unique Constraint |
| </b></p><div class="example-contents"> |
| |
| <p> |
| The following defines a unique constraint on the <code class="literal"> TITLE</code> |
| column of the <code class="literal">ART</code> table: |
| </p> |
| <pre class="programlisting"> |
| @Entity |
| @Table(name="ART", uniqueConstraints=@UniqueConstraint(name="TITLE_CNSTR", columnNames="TITLE")) |
| public class Article { |
| ... |
| } |
| </pre> |
| <p> |
| The same metadata expressed in XML form: |
| </p> |
| <pre class="programlisting"> |
| <entity class="org.mag.Article"> |
| <table name="ART"> |
| <unique-constraint> |
| <name>TITLE_CNSTR</name> |
| <column-name>TITLE</column-name> |
| </unique-constraint> |
| </table> |
| ... |
| </entity> |
| </pre> |
| </div></div><br class="example-break"> |
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