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<p><a id="BABEIEGD" name="BABEIEGD"></a></p>
<div class="sect2"><!-- infolevel="all" infotype="General" -->
<h1>Many-to-many mapping</h1>
<p><a id="sthref130" name="sthref130"></a><a id="sthref131" name="sthref131"></a><a id="sthref132" name="sthref132"></a><a id="sthref133" name="sthref133"></a>Use a <span class="bold">Many-to-Many Mapping</span> to define a many-valued association with many-to-many multiplicity. A many-to-many mapping has two sides: the <span class="italic">owning side</span> and <span class="italic">non-owning side</span>. You must specify the join table on the owning side. For bidirectional mappings, either side may be the owning side.</p>
<ol start="1">
<li>
<p>In the <a href="ref_persistence_outline.htm#BABEGGFE">JPA Structure view</a>, select the field to map. The <a href="ref_persistence_map_view.htm#BABIFBAF">JPA Details view (for attributes)</a> displays the properties for the selected.</p>
</li>
<li>
<p>In the <span class="gui-object-title">Map As</span> field, select <span class="gui-object-action">Many-to-Many</span>.</p>
</li>
<li>
<p>Use this table to complete the <a href="ref_mapping_general.htm#CACBHFIJ">General information</a> fields of the <span class="gui-object-title">JPA Details</span> view.</p>
<div class="inftblruleinformal">
<table class="RuleInformal" title="This table lists the fields in the Persistence Properties view for this mapping." summary="This table lists the fields in the Persistence Properties view for this mapping." dir="ltr" border="1" width="100%" frame="border" rules="all" cellpadding="3" cellspacing="0">
<thead>
<tr align="left" valign="top">
<th align="left" valign="bottom" id="r1c1-t9">Property</th>
<th align="left" valign="bottom" id="r1c2-t9">Description</th>
<th align="left" valign="bottom" id="r1c3-t9">Default</th>
</tr>
</thead>
<tbody>
<tr align="left" valign="top">
<td align="left" id="r2c1-t9" headers="r1c1-t9">Map As</td>
<td align="left" headers="r2c1-t9 r1c2-t9">Defines this mapping as a <span class="bold">Many to Many Mapping</span>.
<p>This field corresponds to the <code>@ManyToMany</code> annotation.</p>
</td>
<td align="left" headers="r2c1-t9 r1c3-t9">Many to Many</td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r3c1-t9" headers="r1c1-t9">Target Entity</td>
<td align="left" headers="r3c1-t9 r1c2-t9">The entity to which this attribute is mapped.</td>
<td align="left" headers="r3c1-t9 r1c3-t9">null
<p>You do not need to explicitly specify the target entity, since it can be inferred from the type of object being referenced.</p>
</td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r4c1-t9" headers="r1c1-t9">Fetch</td>
<td align="left" headers="r4c1-t9 r1c2-t9">Defines how data is loaded from the database. See <a href="ref_mapping_general.htm#CACGGGHB">"Fetch Type"</a> for details.
<ul>
<li>
<p>Eager</p>
</li>
<li>
<p>Lazy</p>
</li>
</ul>
</td>
<td align="left" headers="r4c1-t9 r1c3-t9">Lazy</td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r5c1-t9" headers="r1c1-t9">Mapped By</td>
<td align="left" headers="r5c1-t9 r1c2-t9">The database field that owns the relationship.</td>
<td align="left" headers="r5c1-t9 r1c3-t9"><br /></td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r6c1-t9" headers="r1c1-t9">Order By</td>
<td align="left" headers="r6c1-t9 r1c2-t9">Specify the default order for objects returned from a query. See <a href="ref_mapping_general.htm#CACDADIH">"Order By"</a> for details.
<ul>
<li>
<p>No ordering</p>
</li>
<li>
<p>Primary key</p>
</li>
<li>
<p>Custom</p>
</li>
</ul>
</td>
<td align="left" headers="r6c1-t9 r1c3-t9">No ordering</td>
</tr>
</tbody>
</table>
<br /></div>
<!-- class="inftblruleinformal" --></li>
<li>
<p>Use this table to complete the fields in the <a href="reference007.htm#CACBAEBC">Join Table Information</a> area in the <span class="gui-object-title">JPA Details</span> view.</p>
<div class="inftblruleinformal">
<table class="RuleInformal" title="This table lists the fields in the Join Table tab of the Persistence Properties view for this mapping." summary="This table lists the fields in the Join Table tab of the Persistence Properties view for this mapping." dir="ltr" border="1" width="100%" frame="border" rules="all" cellpadding="3" cellspacing="0">
<thead>
<tr align="left" valign="top">
<th align="left" valign="bottom" id="r1c1-t10">Property</th>
<th align="left" valign="bottom" id="r1c2-t10">Description</th>
<th align="left" valign="bottom" id="r1c3-t10">Default</th>
</tr>
</thead>
<tbody>
<tr align="left" valign="top">
<td align="left" id="r2c1-t10" headers="r1c1-t10">Name</td>
<td align="left" headers="r2c1-t10 r1c2-t10">Name of the join table that contains the foreign key column.</td>
<td align="left" headers="r2c1-t10 r1c3-t10">You must specify the join table on the owning side.
<p>By default, the name is assumed to be the primary tables associated with the entities concatenated with an underscore.</p>
</td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r3c1-t10" headers="r1c1-t10">Join Columns</td>
<td align="left" headers="r3c1-t10 r1c2-t10">Select <span class="bold">Override Default</span>, then Add, Edit, or Remove the join columns.</td>
<td align="left" headers="r3c1-t10 r1c3-t10">By default, the name is assumed to be the primary tables associated with the entities concatenated with an underscore.</td>
</tr>
<tr align="left" valign="top">
<td align="left" id="r4c1-t10" headers="r1c1-t10">Inverse Join Columns</td>
<td align="left" headers="r4c1-t10 r1c2-t10">Select <span class="bold">Override Default</span>, then Add, Edit, or Remove the join columns.</td>
<td align="left" headers="r4c1-t10 r1c3-t10">By default, the mapping is assumed to have a single join.</td>
</tr>
</tbody>
</table>
<br /></div>
<!-- class="inftblruleinformal" --></li>
<li>
<p>To add a new Join or Inverse Join Column, click <span class="gui-object-action">Add</span>.</p>
<p>To edit an existing Join or Inverse Join Column, select the field to and click <span class="gui-object-action">Edit</span>.</p>
</li>
</ol>
<p>Eclipse adds the following annotations to the field:</p>
<pre xml:space="preserve" class="oac_no_warn">
@JoinTable(joinColumns=@JoinColumn(name="<span class="italic"><JOIN_COLUMN></span>"),
name = "<span class="italic"><JOIN_TABLE_NAME></span>")
@ManyToMany(cascade=CascadeType.<span class="italic"><CASCADE_TYPE></span>, fetch=FetchType.<span class="italic"><FETCH_TYPE></span>,
targetEntity=<span class="italic"><TARGET_ENTITY></span>, mappedBy = "<span class="italic"><MAPPED_BY></span>")
@OrderBy("<span class="italic"><ORDER_BY></span>")
</pre>
<p> </p>
<img src="img/ngrelt.png" alt="Related task" title="Related task" /><br />
<br />
<a href="task_mapping.htm#BABDGBIJ">Mapping an entity</a>
<p> </p>
<img src="img/ngrelr.png" alt="Related reference" title="Related reference" /><br />
<br />
<a href="ref_persistence_outline.htm#BABEGGFE">JPA Structure view</a><br />
<a href="ref_persistence_map_view.htm#BABIFBAF">JPA Details view (for attributes)</a>
<p> </p>
<img src="img/ngrelc.png" alt="Related concept" title="Related concept" /><br />
<br />
<a href="concept_mapping.htm#BABBDJFI">Understanding OR mappings</a><br />
<a href="concepts001.htm#BABBGFJG">Understanding EJB 3.0 Java Persistence API</a></div>
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