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

Question

I don't think I will ever fully understand fetch joins.

I have a query where I'm attempting to eagerly "inflate" references down two levels.

That is, my A has an optional Collection of Bs, and each B has either 0 or 1 C. The size of the B collection is known to be small (10-20 tops). I'd like to prefetch this graph.

A's B relationship is marked as FetchType.LAZY and is optional. B's relationship to C is also optional and FetchType.LAZY.

I was hoping I could do:

SELECT a 
  FROM A a
  LEFT JOIN FETCH a.bs // look, no alias; JPQL forbids it
  LEFT JOIN a.bs b // "repeated" join necessary since you can't alias fetch joins
  LEFT JOIN FETCH b.c // this doesn't seem to do anything
 WHERE a.id = :id

When I run this, I see that As B collection is indeed fetched (I see a LEFT JOIN in the SQL referencing the table to which B is mapped).

However, I see no such evidence that C's table is fetched.

How can I prefetch all Cs and all Bs and all Cs that are "reachable" from a given A? I can't see any way to do this.

Answer 1

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