Java – JPA subclasses referenced by multiple different parent classes

I encountered the following ORM problems:

I have two grades A and B, and they all have a group of grades 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;
}

My idea is to use mappedsuperclass for C and have two extension classes, each referenced in a or B But from an object - oriented point of view, this is not the best approach, although I can use superclass types to work with them

Is there a better way to implement this model?

Thank you, Benjamin

Solution

If no mapping annotation is specified (that is, joincolumn or jointable), it uses a join table for each association

Therefore, you will have the following table:

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)

If you annotate each set with the joincolumn annotation, the alternative is:

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

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

Therefore, you will have the following table:

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

This is of course described in the documentation