jpa criteria for many to many relationship

Question

I have 2 POJO classes in Java, Answer and Collaborator, in a many-to-many relationship.

class Answer {     @ManyToMany(cascade = CascadeType.ALL)     @JoinTable(name = "ANSWERS_COLLABORATORS", joinColumns = { @JoinColumn(name = "aid") }, inverseJoinColumns = { @JoinColumn(name = "cid") })     private Set<Collaborator> collaborators = new HashSet<Collaborator>(0); }  

Class Answer has a set of Collaborator, but a Collaborator doesn't keep a set of Answer. What I need to do from Hibernate CriteriaQuery is to find the collaborators for an answer given by id.

I have already done this with Hibernate Criteria (org.hibernate.Criteria) using result transformer, but I'm stuck when it comes to using CriteriaQuery, because I don't have a list of answers to give to the join.

Answer 1

It's done, finally...

Here's the code:

public List<Collaborator> getCollaborators(Long answerId) {   CriteriaBuilder cb = entityManager.getCriteriaBuilder();   CriteriaQuery<Collaborator> criteriaQuery = cb.createQuery(Collaborator.class);      Root<Answer> answerRoot = criteriaQuery.from(Answer.class);   SetJoin<Answer, Collaborator> answers = answerRoot.join(Answer_.collaborators);   criteriaQuery.where(cb.equal(answerRoot.get(Answer_.id), answerId));      return entityManager     .createQuery(criteriaQuery.select(answers))     .getResultList(); } 

Answer 2

Using HQL:

You can use this:

Criteria criteria = session.createCriteria(Answer.class); criteria.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY); criteria.createAlias("collaborators", "collaborators"); criteria.add(Restrictions.eq("collaborators.id",desiredCollaboratorId); 

to get all the Answers associated to a certain Collaborator.

And this:

Criteria criteria = session.createCriteria(Answer.class); criteria.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY); criteria.setFetchMode("collaborators", FetchMode.JOIN) criteria.add(Restrictions.idEq(desiredAnswerId)); dsrTrackingCriteria.setProjection(Projections.property("collaborators")); 

To get all Collaborators associated to a certain Answer.

Using JPA2 Criteria API you can do something like:

CriteriaBuilder cb = em.getCriteriaBuilder(); //creted from EntityManager instance  CriteriaQuery<Long> cq = cb.createQuery(Collaborator.class); Root<Answer> rootAnswer = cq.from(Answer.class); Join<Collaborator,Answer> joinAnswerCollaborator = rootAnswer.join("collaborators"); //(or rootAnswer.join(Answer_.collaborators); if you've created the metamodel with JPA2