Question

Let’s say I have two JPA entities:

@Entity
public class EntityA {
  @Id
  private String id;

  @OneToMany
  private List<EntityB> b;

  ...
}

@Entity
public class EntityB {
  @Id
  private String id;

  ...
}

What I would like to accomplish is to define a @NamedQuery that retrieves all EntityB instances that are not referenced from any EntityA.

Naively, I would have tried

@NamedQuery(name = "EntityB.findAllUnassigned", query = "SELECT b FROM EntityB b WHERE NOT b IN (SELECT DISTINCT a.b FROM EntityA a)")

but this ends up generating invalid SQL:

select b0_.id as id1_, b0_.attr, ... from b b0_ where b0_.id not in  (select distinct . from a a1_, a_b ab2_, b b3_ where a1_.id=ab2_.a_id and ab2_.b_id=b3_.id)

Any alternatives?

Answer 1

Weird that this query does not give you an exception. But your b.a reference is not legal in this way; you cannot select the collection itself, you want/need to select the collection elements.

SELECT b FROM EntityB b WHERE b NOT IN (SELECT DISTINCT elements(a.b) FROM EntityA a)

Note however that elements is a Hibernate HQL construct, not JPA.

If you care about JPA portability:

SELECT b FROM EntityB b WHERE b NOT IN (SELECT DISTINCT b2 FROM EntityA a join a.b b2)

* NOT IN is the more normal form, so I used that here as well