Combining an empty Q with a negated Exists un-negates the Exists lookup

[Jaap Roes]

The following test case fails in Django 3.2 and main:

class TestEmptyQExistsCombination(TestCase):
    def test_combine(self):
        q = Q() & Exists(Book.objects.all())
        self.assertFalse(q.negated)  # passes

    def test_combine_negated(self):
        q = Q() & ~Exists(Book.objects.all())
        self.assertTrue(q.negated)  # fails

I noticed this issue trying to work around issue #32651/ #32548.

[Jaap Roes]

Note, in Django 3.1 the code in the previous tests will raise a TypeError, the following test case achieves the same and passes in Django 3.1, main and (from what I understand) Django 3.2.1

class TestEmptyQExistsCombination(TestCase):
    def test_combine(self):
        q = Q() & Q(Exists(Book.objects.all()))
        self.assertFalse(q.children[0].negated)

    def test_combine_negated(self):
        q = Q() & Q(~Exists(Book.objects.all()))
        self.assertTrue(q.children[0].negated)

[Mariusz Felisiak]

This can be fixed by #32632 (see comment).

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In c8b65943:

Fixed #32632, Fixed #32657 -- Removed flawed support for Subquery deconstruction.

Subquery deconstruction support required implementing complex and
expensive equality rules for sql.Query objects for little benefit as
the latter cannot themselves be made deconstructible to their reference
to model classes.

Making Expression @deconstructible and not BaseExpression allows
interested parties to conform to the "expression" API even if they are
not deconstructible as it's only a requirement for expressions allowed
in Model fields and meta options (e.g. constraints, indexes).

Thanks Phillip Cutter for the report.

This also fixes a performance regression in bbf141bcdc31f1324048af9233583a523ac54c94.

[Mariusz Felisiak <felisiak.mariusz@…>]

In d5add5d3:

[3.2.x] Fixed #32632, Fixed #32657 -- Removed flawed support for Subquery deconstruction.

Subquery deconstruction support required implementing complex and
expensive equality rules for sql.Query objects for little benefit as
the latter cannot themselves be made deconstructible to their reference
to model classes.

Making Expression @deconstructible and not BaseExpression allows
interested parties to conform to the "expression" API even if they are
not deconstructible as it's only a requirement for expressions allowed
in Model fields and meta options (e.g. constraints, indexes).

Thanks Phillip Cutter for the report.

This also fixes a performance regression in bbf141bcdc31f1324048af9233583a523ac54c94.

Backport of c8b659430556dca0b2fe27cf2ea0f8290dbafecd from main