Model validation of constraint involving ForeignObject considers only first column

[Jacob Walls]

Discovered during #36580 in ​review.

Similar to #36431, where only the first column of a ForeignObject was considered in values(), only the first column is considered during model validation of constraints.

Composite PK's are not affected, because they raise system checks if you try to use them in a constraint. But ForeignObject has been broken since its introduction in this regard. Reproduced on 5.2, but thus, not a release blocker.

Rough test (needs adjusting to avoid hijacking this model and unnecessarily skipping tests on backends not supporting constraints):

tests/composite_pk/models/tenant.py

@@ -48 +48 @@
     text = models.TextField(default="", blank=True)
     integer = models.IntegerField(default=0)
 
+    class Meta:
+        # TODO: use new model instead
+        required_db_features = {"supports_table_check_constraints"}
+        constraints = [
+            models.CheckConstraint(
+                condition=models.Q(user__lt=(1000, 1000)),
+                name="user_limit",
+            ),
+        ]
+
 
 class Post(models.Model):
     pk = models.CompositePrimaryKey("tenant_id", "id")

tests/composite_pk/test_models.py

@@ -1 +1 @@
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ValidationError
+from django.db import connection
 from django.test import TestCase
+from django.test.utils import CaptureQueriesContext
 
 from .models import Comment, Tenant, Token, User
 
@@ -119 +121 @@
                 self.assertSequenceEqual(ctx.exception.messages, messages)
 
     def test_full_clean_update(self):
-        with self.assertNumQueries(1):
+        with CaptureQueriesContext(connection) as ctx:
+            self.comment_1.full_clean()
+        select_queries = [
+            query["sql"]
+            for query in ctx.captured_queries
+            if "select" in query["sql"].lower()
+        ]
+        self.assertEqual(len(select_queries), 2, select_queries)  # 1 on 5.2.x
+
+    def test_full_clean_update_invalid(self):
+        self.comment_1.tenant_id = 1001
+        with self.assertRaises(ValidationError):
+            self.comment_1.full_clean()
+
+        self.comment_1.tenant_id = 1
+        self.comment_1.user_id = 1001
+        with self.assertRaises(ValidationError):
             self.comment_1.full_clean()
 
     def test_field_conflicts(self):

tests/composite_pk/tests.py

@@ -187 +187 @@
             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"]

---

Notice 1001 only appears in the first query of the test_full_clean_update_invalid.

FAIL: test_full_clean_update_invalid (composite_pk.test_models.CompositePKModelsTests.test_full_clean_update_invalid)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jwalls/django/tests/composite_pk/test_models.py", line 140, in test_full_clean_update_invalid
    with self.assertRaises(ValidationError):
         ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
AssertionError: ValidationError not raised

----------------------------------------------------------------------
(0.000)
SELECT 1 AS "a"
FROM "composite_pk_tenant"
WHERE "composite_pk_tenant"."id" = 1001
LIMIT 1;

args=(1,
      1001);

ALIAS=DEFAULT (0.000)
SELECT 1 AS "a"
FROM "composite_pk_tenant"
WHERE "composite_pk_tenant"."id" = 1
LIMIT 1;

args=(1,
      1);

ALIAS=DEFAULT
----------------------------------------------------------------------
Ran 2 tests in 0.003s

FAILED (failures=1)