Multiple Bags Exception / Searching multiple collections with JPA

Question

I’m having a problem with JPA when I make an appointment.

I’d like to consult Entity A that has a List of Entity B and List of Entity C (I want to consult Entity A bringing the relationships with Entities B and C into one query).

When I try to execute this query the exception of Multiple bags is launched.

Is there any way to do this consultation without having to use @IndexColumn, Fetch.EAGER or change List for Set? (Solutions I found on the internet)

NOTE: I did not put the code here because I just want to know if there is an alternative other than the ones I mentioned earlier.

Answer 1

The most common cause for this error is when you try to make 2 (or more) JOINs using FETCH, each of these JOINis by means of a List.

Let’s take an example. Given the (pseudo) entities:

@Entity
class Entidade1 {
    private List<Entidade2> entidades2;
}

@Entity
class Entidade2 {
    private List<Entidade3> entidades3;
}

The following JPQL will reproduce the problem:

SELECT e1 FROM Entidade1 e1
JOIN FETCH e1.entidades2 e2
JOIN FETCH e2.entidades3 e3

The most common solution is this: changing from List for Set the mapping of entidades2 and entidades3. The solution with EAGER I wouldn’t even consider, such a consequence that it can bring to the application.

Or, if possible, avoid using a JOIN FETCH from another. As far as I know, only Hibernate supports alias in JOIN FETCH, allowing this type of situation and error.

Thus, the solution that I recommend is to consider the use of a VO, bringing only what you need from the 3 entities:

SELECT new br.com.EntidadeVO(e1.id, e1.nome, e2.id, e3.nome) FROM Entidade1 e1
JOIN e1.entidades2 e2
JOIN e2.entidades3 e3