JPA Critera Query count results involving join

Question

I'm using hibernate and the JPA criteria API and trying to create a re-usable utility method to determine how many rows a query will return.

Currently I have this:

Long countResults(CriteriaQuery cq, String alias){
    CriteriaBuilder cb = em().getCriteriaBuilder();
    CriteriaQuery<Long> countQuery = cb.createQuery(Long.class);
    Root ent = countQuery.from(cq.getResultType());
    ent.alias(alias);
    countQuery.select(cb.count(ent));
    Predicate restriction = cq.getRestriction();
    if(restriction != null){
        countQuery.where(restriction);
    }
    return em().createQuery(countQuery).getSingleResult();
}

Which I use like this:

CriteriaBuilder cb = em().getCriteriaBuilder();
CriteriaQuery<User> cq = cb.createQuery(User.class);
Root<User> root = cq.from(modelClass());
root.alias("ct");
cq.select(root);

TypedQuery<User> query = em().createQuery(cq);
long count = countResults(cq, "ct");

And that works fine. However, when I use a more complicated query like

Join<UserThing, Thing> j = root.join(User_.things).join(UserThing_.thing);
cq.where(somePredicate);

My call to countResults() produces exceptions like org.hibernate.hql.internal.ast.InvalidPathException: Invalid path: 'myAlias.name' , <AST>:0:0: unexpected end of subtree , left-hand operand of a binary operator was null

I'm guessing this has something to do with the join, and that I need to alias that somehow, but I've not had any success so far.

Help?

Answer 1

I had the same problem, and I solved with:

CriteriaQuery<Long> countCriteria = cb.createQuery(Long.class);
Root<EntityA> countRoot = countCriteria.from(cq.getResultType());
Set<Join<EntityA, ?>> joins = originalEntityRoot.getJoins();
for (Join<EntityA, ?> join :  joins) {
    countRoot.join(join.getAttribute().getName());
}
countCriteria.select(cb.count(countRoot));
if(finalPredicate != null)
    countCriteria.where(finalPredicate);

TypedQuery<Long> queryCount = entityManager.createQuery(countCriteria);
Long count = queryCount.getSingleResult();

Where

originalEntityRoot is the main root where I did the query with the where clauses.