[6.0.x] Fixed #36730 -- Fixed constraint validation crash for excluded FK attnames.

Regression in e44e8327d3d88d86895735c0e427102063ff5b55.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

Backport of abfa4619fb818ff694c22e962a280673e085239e from main

3 files changed, 39 insertions, 2 deletions

diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 73ab23bdfa..93251ff322 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -51,9 +51,12 @@ class BaseConstraint:
     def _expression_refs_exclude(cls, model, expression, exclude):
         get_field = model._meta.get_field
         for field_name, *__ in model._get_expr_references(expression):
-            if field_name in exclude:
+            if field_name == "pk":
+                field = model._meta.pk
+            else:
+                field = get_field(field_name)
+            if field_name in exclude or field.name in exclude:
                 return True
-            field = get_field(field_name)
             if field.generated and cls._expression_refs_exclude(
                 model, field.expression, exclude
             ):
diff --git a/tests/model_forms/models.py b/tests/model_forms/models.py
index f6c34a3521..83daa13d71 100644
--- a/tests/model_forms/models.py
+++ b/tests/model_forms/models.py
@@ -543,3 +543,22 @@ class ConstraintsModel(models.Model):
                 violation_error_message="Price must be greater than zero.",
             ),
         ]
+
+
+class AttnameConstraintsModel(models.Model):
+    left = models.ForeignKey(
+        "self", related_name="+", null=True, on_delete=models.SET_NULL
+    )
+    right = models.ForeignKey(
+        "self", related_name="+", null=True, on_delete=models.SET_NULL
+    )
+
+    class Meta:
+        required_db_features = {"supports_table_check_constraints"}
+        constraints = [
+            models.CheckConstraint(
+                name="%(app_label)s_%(class)s_left_not_right",
+                # right_id here is the ForeignKey's attname, not name.
+                condition=~models.Q(left=models.F("right_id")),
+            ),
+        ]
diff --git a/tests/model_forms/tests.py b/tests/model_forms/tests.py
index f0334e1e86..129ce56c7a 100644
--- a/tests/model_forms/tests.py
+++ b/tests/model_forms/tests.py
@@ -30,6 +30,7 @@ from django.utils.version import PY314, PYPY
 from .models import (
     Article,
     ArticleStatus,
+    AttnameConstraintsModel,
     Author,
     Author1,
     Award,
@@ -3766,3 +3767,17 @@ class ConstraintValidationTests(TestCase):
         self.assertEqual(
             full_form.errors, {"__all__": ["Price must be greater than zero."]}
         )
+
+    def test_check_constraint_refs_excluded_field_attname(self):
+        left = AttnameConstraintsModel.objects.create()
+        instance = AttnameConstraintsModel.objects.create(left=left)
+        data = {
+            "left": str(left.id),
+            "right": "",
+        }
+        AttnameConstraintsModelForm = modelform_factory(
+            AttnameConstraintsModel, fields="__all__"
+        )
+        full_form = AttnameConstraintsModelForm(data, instance=instance)
+        self.assertFalse(full_form.is_valid())
+        self.assertEqual(full_form.errors, {"right": ["This field is required."]})