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| <title>2. Unique Constraints</title><link rel="stylesheet" href="css/docbook.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.72.0"><link rel="start" href="manual.html" title="Apache OpenJPA User's Guide"><link rel="up" href="jpa_overview_mapping.html" title="Chapter 12. Mapping Metadata"><link rel="prev" href="jpa_overview_mapping.html" title="Chapter 12. Mapping Metadata"><link rel="next" href="jpa_overview_mapping_column.html" title="3. Column"></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">2. |
| Unique Constraints |
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| Mapping Metadata |
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| Unique Constraints |
| </h2></div></div></div><a class="indexterm" name="d0e9518"></a><a class="indexterm" name="d0e9525"></a><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 a single property: |
| </p><div class="itemizedlist"><ul type="disc"><li><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 contraint's |
| columns. |
| </p><div class="example"><a name="jpa_overview_mapping_unq_attrex"></a><p class="title"><b>Example 12.2. |
| 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=@Unique(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> |
| <column-name>TITLE</column-name> |
| </unique-constraint> |
| </table> |
| ... |
| </entity> |
| </pre></div></div><br class="example-break"></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="jpa_overview_mapping.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="jpa_overview_mapping.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="jpa_overview_mapping_column.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 12. |
| Mapping Metadata |
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