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EclipseLink/UserGuide/MOXy/Relationships/Shared Reference/Keys and Foreign Keys/Composite Key
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Mapping Composite Key Relationships
If the objects that you want to map have multi-part keys (i.e., a combination of fields that determines uniqueness), you can use EclipseLink's @XmlKey and @XmlJoinNodes to set up this relationship.
One or more @XmlKey annotations can be used to declare the primary keys in a given class. For a single key, either @XmlID or @XmlKey can be used. For composite primary keys, multiple @XmlKey annotations can be used, or a single @XmlID can be combined with one or more @XmlKey annotations.
In this example, each Employee has one manager but multiple reports, and Employees are uniquely identified by the combination of their id and name fields.
@XmlAccessorType(XmlAccessType.FIELD) public class Employee { @XmlID @XmlAttribute private Integer id; @XmlKey @XmlAttribute private String name; @XmlJoinNodes( { @XmlJoinNode(xmlPath = "manager/@id", referencedXmlPath = "@id"), @XmlJoinNode(xmlPath = "manager/@name", referencedXmlPath = "@name") }) public Employee manager; @XmlJoinNodes( { @XmlJoinNode(xmlPath = "report/@id", referencedXmlPath = "@id"), @XmlJoinNode(xmlPath = "report/@name", referencedXmlPath = "@name") }) public List<Employee> reports = new ArrayList<Employee>(); ... }
The following example shows how to define this mapping information in EclipseLink's OXM metadata format.
... <java-type name="Employee"> <java-attributes> <xml-attribute java-attribute="id" type="java.lang.Integer" xml-id="true"/> <xml-attribute java-attribute="name" type="java.lang.String"/> <xml-element java-attribute="manager" type="mypackage.Employee" xml-idref="true"/> <xml-element java-attribute="reports" type="mypackage.Employee" container-type="java.util.ArrayList" xml-idref="true"/> </java-attributes> </java-type> ...
This would produce the following XML:
<company> <employee id="1" name="Jane Doe"> <report>2</report> <report>3</report> </employee> <employee id="2" name="John Smith"> <manager>1</manager> </employee> <employee id="3" name="Anne Jones"> <manager>1</manager> </employee> </company>
The manager and reports elements contain the IDs of the Employee instances they are referencing.
Using @XmlList
Because the @XmlIDREF annotation is also compatible with the @XmlList annotation, the Employee object could be modeled as:
@XmlAccessorType(XmlAccessType.FIELD) public class Employee { @XmlID @XmlAttribute private Integer id; @XmlAttribute private String name; @XmlIDREF private Employee manager; @XmlIDREF @XmlList private List<Employee> reports; ... }
This would produce the following XML:
<company> <employee id="1" name="Jane Doe"> <reports>2 3</reports> </employee> <employee id="2" name="John Smith"> <manager>1</manager> </employee> <employee id="3" name="Anne Jones"> <manager>1</manager> </employee> </company>