@ManyToMany JPA relation using a 2nd degree relationship in join table and UK

Question

I have this entity model (simplified):

@Entity
class A {
  @Id
  String id;

  Collection<B> bs;
}

@Entity
class B {
  @Id
  String id;

  @ManyToOne
  @JoinColumn(name = "c_id", nullable = false)
  C c;
}

@Entity
class C {
  @Id
  String id;
}

What would be the best approach to add a join table between A and B (many-to-many relation) and enforce a composite UK using A.id and B.c.id?

I tried something like this:

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable
        (name = "a_b",
                joinColumns = @JoinColumn(name = "a_id"),
                inverseJoinColumns = {
                        @JoinColumn(name = "b_id", referencedColumnName = "id"),
                        @JoinColumn(name = "c_id", referencedColumnName = "c_id")},
                uniqueConstraints = @UniqueConstraint(name = "uk_a_c", columnNames = {"a_id", "c_id"}))

but I get a MultipleBagFetchException, which is very strange. It must be related to the fact that c_id is not part of the primary key. If I remove c_id from inverseJoinColumns it works as expected, but it's not what I need.

Answer 1

My final mapping looks like this:

@Entity
@EntityListeners(ABListener.class)
@Table(name = "a_b", uniqueConstraints =
@UniqueConstraint(name = "uk_rel_a_b", columnNames = {"a_id", "c_id"}))
public class AB extends BaseEntity<Long> {

    private Long id;

    @ManyToOne(optional = false)
    @JoinColumn(name = "a_id")
    private A a;

    @ManyToOne(optional = false)
    @JoinColumn(name = "b_id")
    private B b;

    @ManyToOne(optional = false)
    @JoinColumn(name = "c_id")
    private C c;

    public AB(A a, B b) {
        this.a = a;
        this.b = b;
    }
    /* rest of the class */
}

And a listener to handle setting of C:

public class ABListener {

    @PrePersist
    @PreUpdate
    void handleCUpdate(AB ab) {
        if (ab.getB() != null) {
            ab.setC(ab.getB().getC());
        }
    }

}