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<title>Many-to-one mapping</title>
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<p><a id="BABHFAFJ" name="BABHFAFJ"></a></p>
<div class="sect2"><!-- infolevel="all" infotype="General" -->
<h1>Many-to-one mapping</h1>
<p><a id="sthref134" name="sthref134"></a><a id="sthref135" name="sthref135"></a><a id="sthref136" name="sthref136"></a><a id="sthref137" name="sthref137"></a>Use a <span class="bold">Many-to-One</span> mapping to defines a single-valued association to another entity class that has many-to-one multiplicity.</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-One</span>.</p>
</li>
<li>
<p>Use this table to complete the <a href="ref_mapping_general.htm#CACBHFIJ">General information</a> fields JPA Details view.</p>
<div class="inftblruleinformal">
<table class="RuleInformal" title="This table lists the fields ion the General tab for this mapping." summary="This table lists the fields ion the General tab 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-t11">Property</th>
<th align="left" valign="bottom" id="r1c2-t11">Description</th>
<th align="left" valign="bottom" id="r1c3-t11">Default</th>
</tr>
</thead>
<tbody>
<tr align="left" valign="top">
<td align="left" id="r2c1-t11" headers="r1c1-t11">Target Entity</td>
<td align="left" headers="r2c1-t11 r1c2-t11">The entity to which this attribute is mapped.</td>
<td align="left" headers="r2c1-t11 r1c3-t11">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="r3c1-t11" headers="r1c1-t11">Fetch Type</td>
<td align="left" headers="r3c1-t11 r1c2-t11">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="r3c1-t11 r1c3-t11">Eager</td>
</tr>
</tbody>
</table>
<br /></div>
<!-- class="inftblruleinformal" --></li>
<li>
<p>Use this table to complete the fields on the <a href="reference008.htm#CACFCEJC">Join Columns Information</a> tab 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 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-t12">Property</th>
<th align="left" valign="bottom" id="r1c2-t12">Description</th>
<th align="left" valign="bottom" id="r1c3-t12">Default</th>
</tr>
</thead>
<tbody>
<tr align="left" valign="top">
<td align="left" id="r2c1-t12" headers="r1c1-t12">Join Column</td>
<td align="left" headers="r2c1-t12 r1c2-t12"><a id="sthref138" name="sthref138"></a><a id="sthref139" name="sthref139"></a>Specify a mapped column for joining an entity association. This field corresponds to the <code>@JoinColum</code> attribute.
<p>Select <span class="bold">Override Default</span>, then Add, Edit, or Remove the join columns.</p>
</td>
<td align="left" headers="r2c1-t12 r1c3-t12">By default, the mapping is assumed to have a single join.</td>
</tr>
</tbody>
</table>
<br /></div>
<!-- class="inftblruleinformal" --></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>")
@ManyToOne(targetEntity=<span class="italic"><TARGET_ENTITY></span>, fetch=<span class="italic"><FETCH_TYPE></span>,
cascade=<span class="italic"><CASCADE_TYPE></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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