JPA: uni-directional OneToMany question

I have the following classes:

class A{
   @OneToOne(cascade=CascadeType.ALL)
   private B b;
}

class C{
  @ManyToOne
  private A a;    
}

class B{
  @OneToOne
  private A a;

  @MapKey(name = "name")
  @OneToMany(cascade = CascadeType.ALL, ...)
  @JoinColumn(...)
  private Map<String C> cs;
}

How do I have to specify the mapping on B.cs to join where B.a 开发者_如何学Python== C.a?

Is this possible? Or do I have to change the property C.a to C.b? (I would prefer to keep it as it is, as the entity B is just a helper class.) I also tried to change B to @Embeddable, but Map is not supported for embeddables.

JPA requires that all relationships be by Id (the foreign key references the primary key). So, you need to either add a @ManyToOne from C to B. Or, ensure that B's Id is the foreign key to A (add @Id on the @OneToOne from B to A and remove A's other @Id). If B was a subclass of A instead of having a OneToOne this would also work.

If you are using EclipseLink, you can defined more complex criteria for a relationship. You would need to define the OneToMany's foreign keys using a DescriptorCustomizer and the OneToManyMapping API.

I think you can extends B from A.

If this doesn't work for you, maybe you can add a transient property to refer A's id,

@Transient
Integer getId1() {
    return a.getId();
}

and join C using id1 instead of B's primary key.

Edit: This doesn't work.