[5.2.x] Fixed #36704 -- Fixed system check error for proxy model with a composite pk.

Proxy models subclassing a model with a CompositePrimaryKey were
incorrectly reporting check errors because the check that requires only
local fields to be used in a composite pk was evaluated against the proxy
subclass, which has no fields.

To fix this, composite pk field checks are not evaluated against
proxy subclasses, as none of the checks are applicable to proxy
subclasses. This also has the benefit of not double-reporting real check
errors from an invalid superclass pk.

Thanks Clifford Gama for the review.

Backport of 7456494 from main.

Author: h4l

django/db/models/base.py

@@ -1782,7 +1782,7 @@ def _check_composite_pk(cls):
         meta = cls._meta
         pk = meta.pk
 
-        if not isinstance(pk, CompositePrimaryKey):
+        if meta.proxy or not isinstance(pk, CompositePrimaryKey):
             return errors
 
         seen_columns = defaultdict(list)

docs/releases/5.2.8.txt

@@ -16,3 +16,6 @@ Bugfixes
 * Fixed a bug in Django 5.2 where ``QuerySet.first()`` and ``QuerySet.last()``
   raised an error on querysets performing aggregation that selected all fields
   of a composite primary key.
+
+* Fixed a bug in Django 5.2 where proxy models having a ``CompositePrimaryKey``
+  incorrectly raised a ``models.E042`` system check error.

tests/composite_pk/test_checks.py

@@ -268,3 +268,38 @@ class Bar(Foo):
                 ),
             ],
         )
+
+    def test_proxy_model_can_subclass_model_with_composite_pk(self):
+        class Foo(models.Model):
+            pk = models.CompositePrimaryKey("a", "b")
+            a = models.SmallIntegerField()
+            b = models.SmallIntegerField()
+
+        class Bar(Foo):
+            class Meta:
+                proxy = True
+
+        self.assertEqual(Foo.check(databases=self.databases), [])
+        self.assertEqual(Bar.check(databases=self.databases), [])
+
+    def test_proxy_model_does_not_check_superclass_composite_pk_errors(self):
+        class Foo(models.Model):
+            pk = models.CompositePrimaryKey("a", "b")
+            a = models.SmallIntegerField()
+
+        class Bar(Foo):
+            class Meta:
+                proxy = True
+
+        self.assertEqual(
+            Foo.check(databases=self.databases),
+            [
+                checks.Error(
+                    "'b' cannot be included in the composite primary key.",
+                    hint="'b' is not a valid field.",
+                    obj=Foo,
+                    id="models.E042",
+                ),
+            ],
+        )
+        self.assertEqual(Bar.check(databases=self.databases), [])