2.6.14. Derived Identifiers

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JPA 2.0 added support for derived identifiers which allow an entity to borrow the identifier from a many-to-one or one-to-one association.

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<div class="title">Example 123. Derived identifier with `@MapsId`</div>
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<pre class="prettyprint highlight">`@Entity
public class Person  {

    @Id
    @GeneratedValue
    private Long id;

    @NaturalId
    private String registrationNumber;

    public Person() {}

    public Person(String registrationNumber) {
        this.registrationNumber = registrationNumber;
    }

    public Long getId() {
        return id;
    }

    public String getRegistrationNumber() {
        return registrationNumber;
    }
}

@Entity
public class PersonDetails  {

    @Id
    private Long id;

    private String nickName;

    @ManyToOne
    @MapsId
    private Person person;

    public String getNickName() {
        return nickName;
    }

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }
}`</pre>
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In the example above, the `PersonDetails` entity uses the `id` column for both the entity identifier and for the many-to-one association to the `Person` entity.
The value of the `PersonDetails` entity identifier is "derived" from the identifier of its parent `Person` entity.
The `@MapsId` annotation can also reference columns from an `@EmbeddedId` identifier as well.

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The previous example can also be mapped using `@PrimaryKeyJoinColumn`.

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<div id="identifiers-derived-primarykeyjoincolumn" class="exampleblock">
<div class="title">Example 124. Derived identifier `@PrimaryKeyJoinColumn`</div>
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<pre class="prettyprint highlight">`@Entity
public class PersonDetails  {

    @Id
    private Long id;

    private String nickName;

    @ManyToOne
    @PrimaryKeyJoinColumn
    private Person person;

    public String getNickName() {
        return nickName;
    }

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
        this.id = person.getId();
    }
}`</pre>
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Unlike `@MapsId`, the application developer is responsible for ensuring that the identifier and the many-to-one (or one-to-one) association are in sync.

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