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For example, you have this relationship dened in the ejb-jar.xml le:
<relationships>
<ejb-relation>
<description>Order-OrderLine</description>
<ejb-relation-name>Order-OrderLine</ejb-relation-name>
<ejb-relationship-role>
<ejb-relationship-role-name>
Order-has-N-OrderLines
</ejb-relationship-role-name>
<multiplicity>One</multiplicity>
<relationship-role-source>
<ejb-name>OrderEJB</ejb-name>
</relationship-role-source>
<cmr-field>
<cmr-field-name>orderLines</cmr-field-name>
<cmr-field-type>java.util.Collection</cmr-field-type>
</cmr-field>
</ejb-relationship-role>
</ejb-relation>
</relationships>
When a particular Order is loaded, you can load its related OrderLines by adding this to the
sun-cmp-mapping.xml le for the application:
<entity-mapping>
<ejb-name>Order</ejb-name>
<table-name>...</table-name>
<cmp-field-mapping>...</cmp-field-mapping>
<cmr-field-mapping>
<cmr-field-name>orderLines</cmr-field-name>
<column-pair>
<column-name>OrderTable.OrderID</column-name>
<column-name>OrderLineTable.OrderLine_OrderID</column-name>
</column-pair>
<fetched-with>
<default>
</fetched-with>
</cmr-field-mapping>
</entity-mappping>
Now when an Order is retrieved, the CMP engine issues SQL to retrieve all related OrderLines
with a SELECT statement that has the following WHERE clause:
OrderTable.OrderID = OrderLineTable.OrderLine_OrderID
This clause indicates an outer join. These OrderLines are pre-fetched.
EJB PerformanceTuning
Chapter2 • TuningYourApplication 45