使用 JPA Criteria，如何在不获取已加入实体的情况下获取已加入实体的子实体？

Question

On my project I'm using Groovy with Spring Data JPA's Specification 's to construct Hibernate queries.

I can't provide my actual queries but to illustrate my problem let's say I have Building entities, and each Building has Floors and each Floor has both Rooms and Windows.

The behavior I'm attempting to simulate is something like this native SQL query:

SELECT b.*, r.*
FROM building b
INNER JOIN floor f ON b.id = f.building_id
INNER JOIN window w ON f.id = w.floor_id
LEFT OUTER JOIN room r ON f.id = r.floor_id
WHERE w.id = 1;

I have a specification similar to the below:

public class MySpec implements Specification<Building> {
    @Override
    public Predicate toPredicate(final Root<Building> root, final CriteriaQuery<?> query, final CriteriaBuilder cb) {
        final Join floorsJoin = root.join("floors");
        final Join windowsJoin = floorsJoin.join("windows");

        //I'd like to remove this line
        final Fetch floorsFetch = root.fetch("floors"); // <---

        floorsFetch.fetch("rooms", JoinType.LEFT);

        cb.equal(windowsJoin.get("id"), 1L);
    }
}

The line annotated above is my issue. If I leave it, the generated query looks something like this:

SELECT b.*, f2.*, r.*
FROM building b
INNER JOIN floor f ON b.id = f.building_id
INNER JOIN window w ON f.id = w.floor_id
INNER JOIN floor f2 ON b.id = f2.building_id
LEFT OUTER JOIN room r ON f2.id = r.floor_id
WHERE w.id = 1;

(notice the duplicate INNER JOIN of floor and the unneeded f2.* data)

If I remove it, and use the floorsJoin instead to fetch rooms, I get the following Hibernate error:

org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list

The unneeded f2.* data would be OK except I can't replace the above floorsJoin with the floorsFetch because I need to join with the windows table (without fetching windows ) and the Fetch class doesn't have a .join method.

I'm having a difficult time figuring out how I would accomplish what I need while still generating a single query; surely I must be missing something simple.

Any thoughts or advice you could provide would be much appreciated.

Thanks a lot, BJ

Answer 1

Well it's not that simple with the JPA Criteria API. With Hibernate you could simply cast the Fetch to a Join I guess but that's not going to help you that much. I am not sure how you use the specification in this case, but if you could write the query as a whole it could look like the following

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Tuple> cq = cb.createTupleQuery();

Root<Building> root = cq.from(Building.class);
final Join floorsJoin = root.join("floors");
final Join windowsJoin = floorsJoin.join("windows");
final Join roomsJoin = floorsJoin.join("rooms", JoinType.LEFT);

cb.equal(windowsJoin.get("id"), 1L);

cq.multiselect(
    root,
    roomsJoin
);