java - How to join unrelated entities with the JPA Criteria API

Question

Two database tables have a foreign key relationship.

They are mapped to two entities A and B by JPA, but the join columns are manually removed from the entities, so in JPA world classes A and B are not related and you cannot navigate from one to the other through a field/property.

Using the JPA Criteria API, is it possible to create a query which joins the two tables?

All examples I found on internet uses the join column to achieve the goal, but, as stated above, it was removed from the code because most time I'm not interested in the relationship between A and B and I'm afraid about possible overhead.

Answer 1

First: Foreign key relationship are not only for navigating. They mainly serve to ensure that no spurious values are introduced in the relationship. They also may help the database for query optimization. I would advise you to reconsider that.

Anyway, for creating a query that uses several unrelated entities, you need to put them as from (root) entities (as you would do in SQL or JPQL)

SELECT .... FROM Link l, Training t WHERE l.attribute = t.attribute;

Root<Link> rootLink = criteriaQuery.from(Link.class);
Root<Training> rootTraining = criteriaQuery.from(Training.class);
...
criteriaQuery.where(
    criteriaBuilder.equal(rootLink.get(link_.linkAttribute), trainingLink));