没有@EmbeddedId或@IdClass的超级简单复合键

Question

I don't know since when this is possible, but I just created composite keys the way I always wanted: No @EmbeddedId and no @IdClass required!!

• Spring 4.0.0.M3
• Hibernate 4.3.0.Beta4
• JPA 2.1

User.java :

@Entity
public class User {
    @Id
    ...

    @OneToMany(mappedBy="user", cascade=CascadeType.ALL, orphanRemoval=true)
    private List<GroupUser> groupUsers;     
}

Group.java :

@Entity
public class Group {
    @Id
    ...

    @OneToMany(mappedBy="user", cascade=CascadeType.ALL, orphanRemoval=true)
    private List<GroupUser> groupUsers;     
}

GroupUser.java :

@Entity
public class GroupUser implements Serializable {
    @Id
    @ManyToOne
    @JoinColumn(name="Group_id")
    private Group group;

    @Id
    @ManyToOne
    @JoinColumn(name="User_id")
    private User user;

    @Id
    @Column
    private String s;

    @Column
    private int i;
}

And here is what MySQL says:

 USER             GROUP_USER            GROUP
------    -------------------------    -------
 id        User_id  (PK, NOT NULL)       id 
           Group_id (PK, NOT NULL)
           s        (PK, NOT NULL)
           i        (DEFAULT NULL)

GROUP_USER details:

• PRIMARY KEY: User_id, Group_id, s
• INDEX #1 / FOREIGN KEY: User_id
• INDEX #2 / FOREIGN KEY: Group_id

---

I also did some other tests, like this:

@Entity
public class A implements Serializable {

    @Id
    @Column
    String a;

    @Id
    @Column
    String b;

    @Column
    String c;

}

Works like a charm!!

---

Is there still any reason to use @EmbeddedId or @IdClass ?

And: Is this part of JPA 2.1 or an Hibernate feature?

Answer 1

JPA requires a primary key class of some sort be defined for composite primary keys. In practice though, this seems required mostly for the EntityManager find and getReference calls that require a pk class instance. Implementations may have their own pk representation that you can use for these calls, but will not be portable.