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