2.6.4. Composite identifiers with @IdClass

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Modeling a composite identifier using an IdClass differs from using an EmbeddedId in that the entity defines each individual attribute making up the composition.
The IdClass simply acts as a "shadow".

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<div class="title">Example 111. Basic IdClass</div>
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<pre class="prettyprint highlight">`@Entity
@IdClass( PK.class )
public class Login {

    @Id
    private String system;

    @Id
    private String username;

    public static class PK implements Serializable {

        private String system;

        private String username;

        ...
    }

    ...
}`</pre>
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Non-aggregated composite identifiers can also contain ManyToOne attributes as we saw with aggregated ones (still non-portably)

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<div class="title">Example 112. IdClass with ManyToOne</div>
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<pre class="prettyprint highlight">`@Entity
@IdClass( PK.class )
public class Login {

    @Id
    @ManyToOne
    private System system;

    @Id
    private String username;

    public static class PK implements Serializable {

        private System system;

        private String username;

        ...
    }

    ...
}`</pre>
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With non-aggregated composite identifiers, Hibernate also supports "partial" generation of the composite values.

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<div class="title">Example 113. IdClass with partial generation</div>
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<pre class="prettyprint highlight">`@Entity
@IdClass( PK.class )
public class LogFile {

    @Id
    private String name;

    @Id
    private LocalDate date;

    @Id
    @GeneratedValue
    private Integer uniqueStamp;

    public static class PK implements Serializable {

        private String name;

        private LocalDate date;

        private Integer uniqueStamp;

        ...
    }

    ...
}`</pre>
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This feature exists because of a highly questionable interpretation of the JPA specification made by the SpecJ committee.
Hibernate does not feel that JPA defines support for this, but added the feature simply to be usable in SpecJ benchmarks.
Use of this feature may or may not be portable from a JPA perspective.

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