Joining two tables in JPA when the entities does not have relation mapping between them

Question

I have a User entity and a Pet entity. There are no @OneToMany, @ManyToMany or other between the User and the Pet. The reason is that each user may have thousands of pets (at least in this example). Since I don't want to eager load it (too slow) and I don't want to lazy load (cause lazy initialization exception...and I don't want to use open in view filter) I have created methods in the user repository that have a name like this (the reason is that I want to have the opportunity to return only subsets of the relational properties. I don't want all the pets of a user:

getPetsForUser(final User user){...} . The examples of joins I have seen in JPA obviously uses the relational property on the entities it seems. How do I do this in JPA?

In SQL I would just use the foreign key...

Answer 1

You'll have to explicitly use the foreign key, probably by extracting it from the User object ( User.getId() , I presume) and then using it as a raw long or equivalent field on the Pet --with no foreign-key integrity, cascades, or query-based joins.

You might be better off reexamining why getting lazy-loading working is such a bad thing, and if JPA is the right approach.

Answer 2

JPQL only works on mapped entity and mapped relationships.
One way is to use native SQL query and manage result as List<Object[]> .
Else you can unwrap EntityManager to obtain a Hibernate Session and mappign result with SQLQuery.setResultTransformer(new AliasToBeanResultTransformer(Pet.class)) .
For example:

List<Pet> getPetsForUser(final User user) {
    Session s = entityManager.unwrap(Session.class);
    List<Pet> pets = s.createSQLQuery("select * from PETS p inner join USERS u on (p.user_id = u.id) where u.id = :id").setParameter("id",user.getUserID()).setResultTransformer(new AliasToBeanResultTransformer(Pet.class)).list();
}