带有复合键的 @OneToOne 实体上的 JPA @Id

Question

I have an @Entity class, with an @Id annotation and a @OneToOne annotation on the same field. Usually this would not be a problem, but the entity class of the field with these annotations uses a composite key. This is causing more complications than I anticipated.

Here is the entity class that is posing the problem:

@Entity
public class ReportDetails implements Serializable {

    @Id
    @OneToOne
    private MachineLine machineLine;
}

And here is the MachineLine entity class that is being used as an ID in ReportDetails :

@Entity
@IdClass(MachineLine.MachineLineKey.class)
public class MachineLine {

    @Id
    @ManyToOne
    private Machine machine;

    @Id
    private long lineIndex;

    public static class MachineLineKey implements Serializable {
        private Machine machine;
        private long lineIndex;
    }
}

I have left out any extra fields and the getters and setters from these class definitions, to save space.

When I try to run my application it gives the following exception:

java.lang.IllegalArgumentException: This class [class ReportDetails] does not define an IdClass

When I put an @IdClass annotation on ReportDetails it then requires defining the individual fields of whatever class I define in @IdClass , like in MachineLine . However, I am trying to avoid doing this, in favour of having the whole MachineLine entity returned whenever a ReportDetails entity is retrieved from the database.

Is there a way of having MachineLine as the ID field of ReportDetails , without having to define extra fields within ReportDetails ?

Answer 1

This is what JPA calls a "derived identity". You might try something like this:

ReportDetails:

@Entity
public class ReportDetails implements Serializable {
    // all attributes map by the relationship: AttributeOverride is not allowed
    @EmbeddedId
    private MachineLine.Id id;

    @MapsId
    @JoinColumns({
        @JoinColumn(name="machineId", referencedColumnName="machineId"),
        @JoinColumn(name="machineLineIndex", referencedColumnName="index")
    })
    @OneToOne
    private MachineLine machineLine;

    // ...
}

MachineLine:

@Entity
public class MachineLine {

    @EmbeddedId
    private Id id;

    @MapsId("machineId") // maps machineId attribute of embedded id
    @ManyToOne
    private Machine machine;

    // ...

    @Embeddable
    public static class Id implements Serializable {
        private long machineId; // corresponds to PK type of Machine
        private long index;
        // ...
    }
}

Machine:

@Entity
public class Machine {

    @Id
    private long id;

    @OneToMany(mappedBy = "machine")
    private List<MachineLine> lines;

    // ...
}

Derived identities are discussed (with examples) in the JPA 2.2 spec in section 2.4.1.