Model validation of constraints fails if condition's Q object references ForeignObject

[Jacob Walls]

Similar to #36433, just for ForeignObject instead of ForeignKey.

With this adjusted test model and corresponding adjustment to unrelated test, test_full_clean_update passes on stable/5.2.x (the constraint is purposefully not very imaginative, can polish in PR review):

tests/composite_pk/models/tenant.py

@@ -48 +48 @@
     text = models.TextField(default="", blank=True)
     integer = models.IntegerField(default=0)
 
+    class Meta:
+        constraints = [
+            models.CheckConstraint(
+                condition=models.Q(user__isnull=False),
+                name="user_not_null",
+            ),
+        ]
+
 
 class Post(models.Model):
     pk = models.CompositePrimaryKey("tenant_id", "id")

tests/composite_pk/tests.py

@@ -205 +205 @@
             self.assertEqual(user.email, self.user.email)
 
     def test_select_related(self):
-        Comment.objects.create(tenant=self.tenant, id=2)
+        user2 = User.objects.create(
+            tenant=self.tenant,
+            id=2,
+            email="user0002@example.com",
+        )
+        Comment.objects.create(tenant=self.tenant, id=2, user=user2)
         with self.assertNumQueries(1):
             comments = list(Comment.objects.select_related("user").order_by("pk"))
             self.assertEqual(len(comments), 2)
             self.assertEqual(comments[0].user, self.user)
-            self.assertIsNone(comments[1].user)
+            self.assertEqual(comments[1].user, user2)
 
     def test_model_forms(self):
         fields = ["tenant", "id", "user_id", "text", "integer"]

... but fails on main:

======================================================================
ERROR: test_full_clean_update (composite_pk.test_models.CompositePKModelsTests.test_full_clean_update)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/unittest/case.py", line 651, in run
    self._callTestMethod(testMethod)
    
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/unittest/case.py", line 606, in _callTestMethod
    if method() is not None:
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/tests/composite_pk/test_models.py", line 123, in test_full_clean_update
    self.comment_1.full_clean()
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/base.py", line 1638, in full_clean
    self.validate_constraints(exclude=exclude)
    ^^^^^^^
  File "/Users/jwalls/django/django/db/models/base.py", line 1586, in validate_constraints
    constraint.validate(model_class, self, exclude=exclude, using=using)
    ^^^
  File "/Users/jwalls/django/django/db/models/constraints.py", line 212, in validate
    if not Q(self.condition).check(against, using=using):
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/query_utils.py", line 176, in check
    query.add_q(Q(Coalesce(self, True, output_field=BooleanField())))
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1670, in add_q
    clause, _ = self._add_q(q_object, can_reuse)
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1702, in _add_q
    child_clause, needed_inner = self.build_filter(
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1541, in build_filter
    condition = filter_expr.resolve_expression(
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/expressions.py", line 301, in resolve_expression
    expr.resolve_expression(query, allow_joins, reuse, summarize, for_save)
    ^^^^^^^
  File "/Users/jwalls/django/django/db/models/query_utils.py", line 91, in resolve_expression
    clause, joins = query._add_q(
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1702, in _add_q
    child_clause, needed_inner = self.build_filter(
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1527, in build_filter
    return self._add_q(
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1702, in _add_q
    child_clause, needed_inner = self.build_filter(
    ^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1550, in build_filter
    lookups, parts, reffed_expression = self.solve_lookup_type(arg, summarize)
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1357, in solve_lookup_type
    _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
    ^^^^^^^^^^^^^^^
  File "/Users/jwalls/django/django/db/models/sql/query.py", line 1830, in names_to_path
    raise FieldError(
    ^^^
django.core.exceptions.FieldError: Cannot resolve keyword 'user' into field. Choices are: _check

----------------------------------------------------------------------
Ran 178 tests in 1.224s

---

There is a prior comment in ticket:36433#comment:1 about improving the error message.

[Jacob Walls]

I neglected to check a test project, there's a system check preventing you from getting into this situation that errors on migrate:

ERRORS:
models.OrderLineItem: (models.E012) 'constraints' refers to the nonexistent field 'user'.

So this needs a more realistic test case to find out if there's a usage of Q() supported on 5.2 that is actually breaking here.

[JaeHyuckSa]

Replying to Jacob Walls:

I neglected to check a test project, there's a system check preventing you from getting into this situation that errors on migrate:

ERRORS:
models.OrderLineItem: (models.E012) 'constraints' refers to the nonexistent field 'user'.

So this needs a more realistic test case to find out if there's a usage of Q() supported on 5.2 that is actually breaking here.

Hi Jacob

Is there a way to run the system checks inside the Django source repository rather than in my own project?

[Jacob Walls]

Reopening, my constraint didn't match my models. Sorry for the noise!

Is there a way to run the system checks inside the Django source repository rather than in my own project?

I'm not sure we run system checks on test models. Not super relevant to this ticket, as I was just totally wrong in this case, but I have noticed it when working in the test suite that test models might not be 100% valid.

[JaeHyuckSa]

Replying to Jacob Walls:

Reopening, my constraint didn't match my models. Sorry for the noise!

Is there a way to run the system checks inside the Django source repository rather than in my own project?

I'm not sure we run system checks on test models. Not super relevant to this ticket, as I was just totally wrong in this case, but I have noticed it when working in the test suite that test models might not be 100% valid.

I wasn’t aware of that. I’ll keep it in mind in future tests. Thank you for your response.