CriteriaBuilder.isEmpty on an ElementCollection vs. JPQL approach - api

Question

I'm trying to do a simple query using the JPA2 criteria API on the following class(es):
// a lot of imports
#Entity
public class Thing {
enum Type { FIRST, SECOND, THIRD };
#SequenceGenerator(name = "Thing_SeqGen", sequenceName = "Thing_Id_Seq", initialValue = 1000)
#Id
#GeneratedValue(generator = "Thing_SeqGen")
private int id;
private String name = "name";
#Enumerated(EnumType.STRING)
#ElementCollection(targetClass = Thing.Type.class)
#CollectionTable(name = "TYPES", joinColumns = { #JoinColumn(referencedColumnName = "ID", name = "TYPE_ID") })
private Set<Thing.Type> typeSet = new HashSet<Thing.Type>();
public static void main(final String[] args) {
new Thing().start();
}
public void start() {
final Thing firstThing = new Thing();
firstThing.setName("First one");
firstThing.setTypeSet(EnumSet.of(Thing.Type.FIRST));
final Thing firstAndSecondThing = new Thing();
firstAndSecondThing.setName("Test2");
firstAndSecondThing.setTypeSet(EnumSet.of(Thing.Type.FIRST, Thing.Type.SECOND));
final Thing bareThing = new Thing();
bareThing.setName("Test3");
final EntityManagerFactory emf = Persistence.createEntityManagerFactory("sandbox");
final EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
em.persist(firstThing);
em.persist(firstAndSecondThing);
em.persist(bareThing);
em.getTransaction().commit();
em.getTransaction().begin();
final CriteriaBuilder cb = em.getCriteriaBuilder();
final CriteriaQuery<Thing> c = cb.createQuery(Thing.class);
final Root<Thing> root = c.from(Thing.class);
final Join<Thing, Set<Thing.Type>> typeJoin = root.join("typeSet");
c.select(root).distinct(true).where(cb.isEmpty(typeJoin));
final List<Thing> results = em.createQuery(c).getResultList();
em.getTransaction().commit();
}
// getter/setter methods omitted
}
What I want to query: Find all things which has no typeset.
The JPQL which does the job is:
select t from Thing t where t.typeSet is empty
The JPQL query returns one result which is expected. The criteria query returns no results. The CriteriaBuilder created:
SELECT DISTINCT t0.ID, t0.NAME FROM THING t0, TYPES t1 WHERE (((SELECT COUNT(t2.ID) FROM THING t2 WHERE (t1.TYPE_ID = t0.ID)) = 0) **AND (t1.TYPE_ID = t0.ID)**)
The last theta-join (marked **) kills it all. And I have no idea why the table THING is specified twice (THING to, THING t1).
Obviously I'm doing wrong. But I have no clue what's the fault.

Answer 1

I'd guess the problem is that you're trying to do an explicit join in the Criteria case, whereas in the JPQL you don't. So omit the join and do something like
Metamodel model = emf.getMetamodel();
ManagedType thingType = model.managedType(Thing.class);
CollectionAttribute typeSetAttr = thingType.getCollection("typeSet");
c.select(root).distinct(true).where(cb.isEmpty(root.get(typeSetAttr)));
This should then translate into the same JPQL as you posted ... or at least it does for DataNucleus JPA implementation.