Why is where clause omitted in jpa Criteria Query with JOINS? - jpa-2.0

Question

I clearly defined a WHERE statements in the main and sub-query. This is printed in the JPA log if I omit the subquery. However, the same where clause ist not applied, if I add the subquery. What is it about JPA Criteria query, that omits the WHERE clause (selection σ)?
// 1) MainQuery
// Create the FROM
Root<PubThread> rootPubThread = cq.from(PubThread.class);
// Create the JOIN fro the first select: join-chaining. You only need the return for ordering. e.g. cq.orderBy(cb.asc(categoryJoin.get(Pub_.title)));
Join<Pub, PubCategory> categoryJoin = rootPubThread.join(PubThread_.pups).join(Pub_.pubCategory);
// Create the WHERE
cq.where(criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)));
// Create the SELECT, at last
cq.select(rootPubThread).distinct(true);
// 2) Subquery
Subquery<PubThread> subquery = cq.subquery(PubThread.class);
Root<PubThread> rootPubThreadSub = subquery.from(PubThread.class);
subquery.where(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId));
Join<Pub, PubCategory> categoryJoinSub = rootPubThreadSub.join(PubThread_.pups).join(Pub_.pubCategory);
subquery.select(rootPubThreadSub);
Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
subquery.where(correlatePredicate);
cq.where(criteriaBuilder.exists(subquery));
Here is the output of this query:
select distinct
pubthread0_.id as id3_,
pubthread0_.dateCreated as dateCrea2_3_,
pubthread0_.dateModified as dateModi3_3_,
pubthread0_.name as name3_
from
pubthread pubthread0_
inner join
pub_pubthread pups1_ ON pubthread0_.id = pups1_.pubThreads_id
inner join
pub pub2_ ON pups1_.pups_id = pub2_.id
inner join
PubCategory pubcategor3_ ON pub2_.pubCategoryId = pubcategor3_.id
where
exists( select
pubthread4_.id
from
pubthread pubthread4_
inner join
pub_pubthread pups5_ ON pubthread4_.id = pups5_.pubThreads_id
inner join
pub pub6_ ON pups5_.pups_id = pub6_.id
inner join
PubCategory pubcategor7_ ON pub6_.pubCategoryId = pubcategor7_.id
where
pubthread4_.id=pubthread0_.id)

Answer 1

Got it. As soon as you defined a predicate, the where clause is rejected. That is for sure. Combine the where clause with a conjunction within a Predicate instance and append it in the where clause. Use an criteriaBuilder.and(..., ...). To semantically do the same thing.
So now, there are two conjunctional predicates: One for the subquery and one for the main query. Notice the obsolete single select(...) I ruled out from the code (it's commented).
CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
CriteriaQuery<PubThread> mainQuery = criteriaBuilder
.createQuery(PubThread.class);
// 1) MainQuery
// Create the FROM
Root<PubThread> rootPubThread = mainQuery.from(PubThread.class);
// Create the JOIN from the first select: join-chaining. You only need the return for ordering. e.g. cq.orderBy(cb.asc(categoryJoin.get(Pub_.title)));
Join<Pub, PubCategory> categoryJoin = rootPubThread.join(PubThread_.pups).join(Pub_.pubCategory);
// Create the WHERE
mainQuery.where(criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)));
// Create the SELECT, at last
mainQuery.select(rootPubThread).distinct(true);
// 2) Subquery
Subquery<PubThread> subquery = mainQuery.subquery(PubThread.class);
Root<PubThread> rootPubThreadSub = subquery.from(PubThread.class);
//subquery.where(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId));
Join<Pub, PubCategory> categoryJoinSub = rootPubThreadSub.join(PubThread_.pups).join(Pub_.pubCategory);
subquery.select(rootPubThreadSub);
//Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
Predicate correlatePredicate = criteriaBuilder.and(
criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread),
criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)
);
subquery.where(correlatePredicate);
//Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
Predicate mainPredicate = criteriaBuilder.and(
criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)),
criteriaBuilder.exists(subquery)
);
//cq.where(criteriaBuilder.exists(subquery));
mainQuery.where(mainPredicate);
TypedQuery<PubThread> typedQuery = em.createQuery(mainQuery);
List<PubThread> otherPubThreads = typedQuery.getResultList();
return otherPubThreads;
The result is something like this
where
pubthread4_.id=pubthread0_.id
and pubthread0_.id=1