CriteriaBuilder cb = entityManager.getCriteriaBuilder();
 CriteriaQuery<Long> cq = cb.createQuery(Long.class);
 Root<Foo> from = cq.from(Foo.class);
 MapJoin<Foo, Bar, Long> joinMap = from.joinMap("map");
 Path<Bar> key = joinMap.key();
 Path<Long> id = key.get("id");
 cq.select(id);
 entityManager.createQuery(cq).getResultList();