Foreign key mapping is the most common mapping style for one-to-one and one-to-many relationships, but is not allowed for many-to many relationships. The foreign key mapping element is simply declared by adding an empty foreign key-mapping element to the ejb-relation element.

As noted in the previous section, with a foreign key mapping the key-fields declared in the ejb-relationship-role are added to the table of the related entity. If the key-fields element is empty, a foreign key will not be created for the entity. In a one-to-many relationship, the many side (Gangster in the example) must have an empty key-fields element, and the one side (Organization in the example) must have a key-fields mapping. In one-to-one relationships, one or both roles can have foreign keys.

The foreign key mapping is not dependent on the direction of the relationship. This means that in a one-to-one unidirectional relationship (only one side has an accessor) one or both roles can still have foreign keys. The complete foreign key mapping for the Organization-Gangster relationship is shown below with the foreign key elements highlighted in bold:

<jbosscmp-jdbc>
    <relationships>
        <ejb-relation>
            <ejb-relation-name>Organization-Gangster</ejb-relation-name>
            
               <foreign-key-mapping/>
            
            <ejb-relationship-role>
                <ejb-relationship-role-name>org-has-gangsters</ejb-relationship-role-name>
                
               <key-fields> <key-field> <field-name>name</field-name> <column-name>organization</column-name> </key-field> </key-fields>
            
            </ejb-relationship-role>
            <ejb-relationship-role>
                <ejb-relationship-role-name>gangster-belongs-to-org</ejb-relationship-role-name>
                
               <key-fields/>
            
            </ejb-relationship-role>
        </ejb-relation>
    </relationships>
</jbosscmp-jdbc>