Reusing same instance of FilteredRelations in multiple queries result in django.core.exceptions.FieldError: Cannot resolve keyword

[Viliam Mihálik]

Example test case that should pass, used in FilteredRelationTests

def test_reuse_same_filtered_relation(self):
    borrower = Borrower.objects.create(name="Jenny")
    Reservation.objects.create(
        borrower=borrower,
        book=self.book1,
        state=Reservation.STOPPED,
    )
    condition = Q(book__reservation__state=Reservation.STOPPED)
    my_reserved_books = FilteredRelation(
        "book__reservation",
        condition=condition
    )
    first_result = list(Author.objects.annotate(
        my_reserved_books=my_reserved_books,
    ))
    self.assertEqual(my_reserved_books.condition, condition)
    # AssertionError: <Q: (AND: ('my_reserved_books__state', 'stopped'))> != <Q: (AND: ('book__reservation__state', 'stopped'))>
    second_result = list(Author.objects.annotate(
        my_reserved_books=my_reserved_books,
    ))
    # django.core.exceptions.FieldError: Cannot resolve keyword 'my_reserved_books' into field. Choices are: book, content_object, content_type, content_type_id, favorite_books, id, name, object_id

When you reuse same FilteredRelation in different queries, internal FilteredRelation.condition is changed and no longer works.
This same code works on Django 4.x. This change was introduced by fixing of bug #33766

Based on discussion in #33766 it was intended but breaking for us after upgrading from 4 -> 5.
In order to get this working is to my_reserved_books.clone() clone annotate before using.

[Simon Charette]

We should definitely clone the filtered relation on resolving, the bug is present since 5.0 though so it doesn't qualify for a backport.

Would you be interested in submitting a patch here? I suspect adjusting FilteredRelation.clone to make sure it passes condition=self.condition.clone() should do?

[Viliam Mihálik]

I can look at it.
I checked the code, and after debug, .clone method is never called. So my suggestion is to clone annotation before "rename" here
​https://github.com/django/django/blob/main/django/db/models/query.py#L1728

and simply change clone.query.add_filtered_relation(annotation, alias) to clone.query.add_filtered_relation(annotation.clone(), alias)

[Sarah Boyce <42296566+sarahboyce@…>]

In bd65e828:

Fixed #36442 -- Cloned FilteredRelation before rename_prefix_from_q.