How do I do a "deep" fetch join in JPQL?

JPA does not allow nested join fetches, nor allow an alias on a join fetch, so this is probably JPA provider specific.

In EclipseLink, you can specify a query hint to perform nested join fetches.

You can't make it recursive in JPQL though, you could go only at best n levels. In EclipseLink you could use @JoinFetch or @BatchFetch on the mapping to make the querying recursive.

See, http://java-persistence-performance.blogspot.com/2010/08/batch-fetching-optimizing-object-graph.html

Source: http://www.coderanch.com/t/570828/ORM/databases/Recursive-fetch-join-recursively-fetching

I'm using Hibernate (and this may be specific to it) and I've had success with this:

SELECT DISTINCT a, b 
FROM A a
LEFT JOIN a.bs b
LEFT JOIN FETCH a.bs
LEFT JOIN FETCH b.c
WHERE a.id = :id

(Note the b in the select list).

This was the only way I found this would work for me, note that this returns Object[] for me and I then filter it in code like so:

(List<A>) q.getResultList().stream().map(pair -> (A) (((Object[])pair)[0])).distinct().collect(Collectors.toList());

Not exactly JPQL, but you can achieve that in pure JPA with Criteria queries:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<MyEntity> q = cb.createQuery(MyEntity.class);
Root<MyEntity> root = q.from(MyEntity.class);
q.select(root);
Fetch bsFetch = root.fetch("b", JoinType.LEFT); //fetch b, property of MyEntity and hold fetch object
bsFetch.fetch("c", JoinType.LEFT); //fetch c, property of b

Support for this kind of nested fetch is vendor-specific (as JPA doesn't require them to do so), but both eclipselink and hibernate do support it, and this way your code remains vendor independant.

The JPA spec does not allow aliasing a fetch join, but some JPA providers do.

EclipseLink does as of 2.4. EclipseLink also allow nested join fetch using the dot notation (i.e. "JOIN FETCH a.bs.c"), and supports a query hint "eclipselink.join-fetch" that allows nested joins (you can specify multiple hints of the same hint name).

In general you need to be careful when using an alias on a fetch join, as you can affect the data that is returned.

See, http://java-persistence-performance.blogspot.com/2012/04/objects-vs-data-and-filtering-join.html