Question

I am trying to use subqueries in an application I am writing using JPA 2.0 type-safe criteria API, with Hibernate 3.6.1.Final as my provider. I have no problem selecting primitive types (Long, MyEntity, etc.), but I want to select multiple columns.

Here’s an example of something completely reasonable. Ignore the needless use of subquery — it is simply meant as illustrative.

    EntityManager em = getEntityManager();
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<Tuple> cq = cb.createTupleQuery();

    Subquery<Tuple> subQ = cq.subquery(Tuple.class);
    Expression<Long> subqCount;
    {
        Root<MyEntity> root = subQ.from(MyEntity.class);
        Path<MyEntity> filter = root.get(MyEntity.challenge);

        subqCount = cb.count(root);

        // How to select tuple?
        Selection<Tuple> tuple = cb.tuple(filter, subqCount);

                    // !! Run-time exception here
        Expression<Tuple> tupleExpr = (Expression<Tuple>) tuple; 

        // Not sure why I can't use multiSelect on a subQuery
        // #select only accepts Expression<Tuple>
        createSubQ.select(tupleExpr);

        createSubQ.groupBy(filter);
    }

    cq.multiselect(subqCount);

Although the compiler doesn’t complain, I still get a run-time exception.

    java.lang.ClassCastException: org.hibernate.ejb.criteria.expression.CompoundSelectionImpl cannot be cast to javax.persistence.criteria.Expression

• Is this a bug in hibernate, or am I doing something wrong?
• If you can’t use multiselect on a subquery, then how can you perform a groupBy?
• If you can’t use groupBy on a subquery, why is it in the API?

Answer 1

I have the same problem.

I can only attempt to answer your last question by saying you can only really use sub queries to perform very simple queries like:

SELECT name FROM Pets WHERE Pets.ownerID in (
    SELECT ID FROM owners WHERE owners.Country = "SOUTH AFRICA"
)

The other thing I wanted to say was how much this incident reminds me of xkcd #979.