Hi all,
My earlier message skipped the background, so here it is first.
Aliases in GORM criteria: what a query means when it names a collection twice
A criteria query can reach an association in two ways: an alias,
createAlias('scores', 'a'), which names a join so a restriction can say
a.round, or a block, scores { eq('round', 'first') }. Each one is a join of the
association. The open question is what happens when one query uses several.
Take an owner with a score in round one and another in round two:
Pet.withCriteria {
owner { scores { eq('round', 'first') } }
owner { scores { eq('round', 'second') } }
}
If the two blocks share a join, one score has to be in both rounds, so no pet
matches. If each has its own join, the pet matches. Hibernate 5 shares them.
Hibernate 7 on 8.0.x shared them in most shapes, but not in this nested one.
Hibernate 5 also contradicts itself: for the same criteria it can return
different rows depending on statement order, or on whether the criteria runs at
the top or inside a subquery. It throws for 71% of the combinations we tested
and contradicts itself for 7%, so it cannot be the reference.
A DetachedCriteria also dropped the first alias when a second was declared for
the same association (#16851).
The rule, decided in https://github.com/apache/grails-core/issues/16889: blocks
without an alias share one join. The first alias of an association path shares
it, and every later alias has its own join, in the same block or another. An
order by a path in a block that declares an alias for it sorts by that alias.
To ask for two different elements, use two aliases.
The PRs: four open PRs (#16773, #16866, #16863 and #16559) collided in the same
files, so they are consolidated with every author's commits kept:
https://github.com/apache/grails-core/pull/16891 for Hibernate 7 on 8.0.x, and
https://github.com/apache/grails-core/pull/16890 for the shared core part on
7.0.x, which merges first. The core change is opt-in per datastore, so Mongo,
Neo4j and the simple datastore behave as before. #16853 stays on its own. The
suites and style checks pass, and the originals stay open until these are
reviewed.
Any objection to the rule?
Walter
