#36109: Three or more stacked FilteredRelations can create a RecursionError
-------------------------------------+-------------------------------------
Reporter: Peter DeVita | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 5.1 | Severity: Normal
Keywords: FilteredRelation | Triage Stage:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Example test case, this belongs to FilteredRelationTests
{{{
def test_three_level_nested_chained_relations(self):
borrower1 = Borrower.objects.create(name="Jenny")
# borrower 1 reserves, rents, and returns book1.
Reservation.objects.create(
borrower=borrower1,
book=self.book1,
state=Reservation.STOPPED,
)
qs = (
Author.objects.annotate(
my_books=FilteredRelation(
"book"
),
my_reserved_books=FilteredRelation(
"my_books__reservation",
condition=Q(my_books__reservation__state=Reservation.STOPPED)
),
my_readers=FilteredRelation(
"my_reserved_books__borrower",
condition=Q(my_reserved_books__borrower=borrower1)
)
)
)
qs = qs.filter(
my_readers=borrower1
)
self.assertQuerySetEqual(
qs,
[
("Alice",)
],
lambda x: (x.name,),
)
}}}
This will result in a {{{RecursionError}}} when the filter is applied.
From what I can understand, this is because on {{{Query}}} when it gets to
{{{setup_joins}}} and attempts to setup the join for the {{{my_readers}}}
FilteredRelation, it is not allowed to reuse the aliases from the first
two `FilteredRelation`s. It then constantly recurses, trying to setup the
joins and then not being able to reuse the joins.
I'm no ORM expert but the fix might be as simple as just adding the
FilteredRelation's join alias to the {{{can_reuse}}} set. Any joins that
were going to that table but not through the FilteredRelation should be
unaffected since they weren't going to reference its alias anyways. Any
joins that do reference that FilteredRelation should always want to reuse
this too. The fix is in my diff/PR and tests all pass with it.
This bug might also be only because we're filtering on the last join in
the set as the first filter condition, it might be possible to force the
aliases to be reusable if you filtered one at a time on each
FilteredRelation leading up to it.
--
Ticket URL: <https://code.djangoproject.com/ticket/36109>
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