JPA Child Class Referenced By Multiple Different Parent Classes

Question

I met the following ORM problem:

I am having two classes A and B who both are having a Set of class C:

class A {
  @Id
  @GeneratedValue
  private long id;

  @OneToMany
  private Set<C> cSet;
}

class B {
  @Id
  @GeneratedValue
  private long id;

  @OneToMany
  private Set<C> cSet;
}

class C {
  @Id
  @GeneratedValue
  private long id;
}

One idea I had was using a MappedSuperclass for C and having two extended classes that are each referenced in either A or B. But from an object oriented point of view this is not the nicest approach although I could use the superclass type in order to work with them.

Is there any nicer way to realize this model?

Thanks, Benjamin

Answer 1

If you don't specify any mapping annotation (i.e. JoinColumn or JoinTable), it will use a join table for each association.

You will thus have the following tables:

A : id
B : id
C : id
A_C : a_id, c_id (where c_id is unique)
B_C : a_id, c_id (where c_id is unique)

The alternative if to annotate each set with a JoinColumn annotation:

class A {
    @OneToMany
    @JoinColumn(name = "a_id")
    private Set<C> cSet;
}

class B {
   @OneToMany
   @JoinColumn(name = "b_id")
   private Set<C> cSet;
}

And you will thus have the following tables:

A : id
B : id
C : id, a_id, b_id

This is of course described in the documentation.