Question

Let’s say I have entities A, B, C and each A has many B and C entities. I want to query a load of A entities based on some criterea, and I know I will be accessing all B and C entities for each A I return.

Something like select a from A as a join fetch a.b join fetch a.c would seem to make sense at first, but this creates a huge product if the numbers of B and C entities are large. Extending this to another associated entities makes the query totally unreasonable.

If I leave JPA to its own devices, I end up with n+1 selects when it wants to access the B and C entities.

What I thought I’d do was query A join fetch B, then A join fetch C, but this doesn’t work as it gives me two List<A> results each with only half the information.

This is a pretty simple query in SQL terms, and I’m disappointed there isn’t an obvious way to handle this. Am I missing something?

Provider is toplink essentials

Answer 1

I wonder why you say this is pretty simple in SQL terms. Wouldn’t you also have the cartesian product?

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Using the Hibernate provider for JPA, an option you mention works:

query A join fetch B, then A join fetch C

You have two list of the same values, you use only one and it is fine (you just need to LEFT join).

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In Hibernate, you can also ask to fetch the missing data in a second query.

Use fetch="subselect".

See https://www.hibernate.org/315.html

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UPDATED after comment of the Original Poster:

In java, you could also do this by hand.

1. Fetch the As with their collections of Bs, in a list called entityAs.
2. Fetch the As with their collections of Cs (reusing part of the query, or using ids).
3. Create a datastructure Map> for the second query (for performance, to avoid inner loop).
4. Loop on list entityAs, using the Map to set the set Cs for each instance A.

This would have a good performance also.

If you run several times into this need, you could write a parameterized method to do this for you, so you only code it once.

As commented by the Original Poster, you need to detach all A entities from entityAs before modifying them, to be sure there will be no update send to the database…