UniqueTogetherValidator fails validating partial unique constraint when there is a nullable FK

[sevdog]

In one of my projects I have a model which defines a set of nullable foreign-keys and a value which is unique according to how FKs are defined, like the following:

class UniqueConstraintWithRelationModel(models.Model):
    value = models.IntegerField()
    first_related = models.ForeignKey(FistModel, related_name='+', on_delete=models.CASCADE, null=True, blank=True)
    second_related = models.ForeignKey(SecondModel, related_name='+', on_delete=models.CASCADE, null=True, blank=True)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=('value', 'first_related'),
                condition=models.Q(second_related__isnull=True),
                name='unique_constraint_with_relationa',
            ),
            models.UniqueConstraint(
                fields=('value', 'second_related'),
                condition=models.Q(first_related__isnull=True),
                name='unique_constraint_with_relationb',
            ),
        ]

When validating this model using a ModelSerializer providing an input which triggers an error on one of those constraint I am not getting any error in serializer.is_valid().

Here is a commit which adds a test case which fails with the current implementation: a4a3093

By debugging the problem seems to be how the query is build, because when executing the same query as in qs_exists_with_condition I get the following error (which is masked by the function and thus the validation prodives a false negative!):

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/query_utils.py", line 135, in check
    query.add_q(Q(Coalesce(self, True, output_field=BooleanField())))
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1609, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1641, in _add_q
    child_clause, needed_inner = self.build_filter(
                                 ^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1482, in build_filter
    condition = filter_expr.resolve_expression(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/expressions.py", line 1073, in resolve_expression
    c.source_expressions[pos] = arg.resolve_expression(
                                ^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/query_utils.py", line 90, in resolve_expression
    clause, joins = query._add_q(
                    ^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1641, in _add_q
    child_clause, needed_inner = self.build_filter(
                                 ^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1509, in build_filter
    condition = self.build_lookup(lookups, reffed_expression, value)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1375, in build_lookup
    lookup_class = lhs.get_lookup(lookup_name)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/expressions.py", line 390, in get_lookup
    return self.output_field.get_lookup(lookup)
           ^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/utils/functional.py", line 47, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
                                         ^^^^^^^^^^^^^^^^^^^
  File "/opt/django-rest-framework/.venv/lib/python3.12/site-packages/django/db/models/expressions.py", line 317, in output_field
    raise FieldError("Cannot resolve expression type, unknown output_field")
django.core.exceptions.FieldError: Cannot resolve expression type, unknown output_field

This problems forces me to rewrite the validation for the constraint which results in an extra query which is performed against the database.

[auvipy]

is this PR related #9766 ?

[sevdog]

That PR does not involves partial constraints, it is just for error message.

I see that that PR does not take into account constraint's condition, since there are no references of this term in the newly added code.

[sevdog]

PS: in my project the setup is a bit more complex because I have 4 FKs (A,B,C,D) which should form two families of unique sets (A,B,C) when D is NULL and (A,B,D) when C is NULL. In my branch I have reduced to two FK with a value for simplicity.

[auvipy]

would you mind come with a PR containing the failing test?

[sevdog]

Here it is #9779.

Has the action logs shows the constraint will fail and yet the serializer, which has every fields and every element is provided in input, validates without errors.