[4.1.x] Fixed #33952 -- Reallowed creating reverse foreign key managers on unsaved instances.

Thanks Claude Paroz for the report.

Regression in 7ba6ebe914.

Backport of 806e9e2d0d from main

django/db/models/fields/related_descriptors.py

@@ -627,15 +627,6 @@ def create_reverse_many_to_one_manager(superclass, rel):
 
             self.core_filters = {self.field.name: instance}
 
-            # Even if this relation is not to pk, we require still pk value.
-            # The wish is that the instance has been already saved to DB,
-            # although having a pk value isn't a guarantee of that.
-            if self.instance.pk is None:
-                raise ValueError(
-                    f"{instance.__class__.__name__!r} instance needs to have a primary "
-                    f"key value before this relationship can be used."
-                )
-
         def __call__(self, *, manager):
             manager = getattr(self.model, manager)
             manager_class = create_reverse_many_to_one_manager(manager.__class__, rel)
@@ -700,6 +691,14 @@ def create_reverse_many_to_one_manager(superclass, rel):
                 pass  # nothing to clear from cache
 
         def get_queryset(self):
+            # Even if this relation is not to pk, we require still pk value.
+            # The wish is that the instance has been already saved to DB,
+            # although having a pk value isn't a guarantee of that.
+            if self.instance.pk is None:
+                raise ValueError(
+                    f"{self.instance.__class__.__name__!r} instance needs to have a "
+                    f"primary key value before this relationship can be used."
+                )
             try:
                 return self.instance._prefetched_objects_cache[
                     self.field.remote_field.get_cache_name()

docs/releases/4.1.1.txt

@@ -49,3 +49,6 @@ Bugfixes
 
 * Fixed a regression in Django 4.1 that caused an incorrect migration when
   renaming a model with ``ManyToManyField`` and ``db_table`` (:ticket:`33953`).
+
+* Reallowed, following a regression in Django 4.1, creating reverse foreign key
+  managers on unsaved instances (:ticket:`33952`).

tests/many_to_one/tests.py

@@ -748,6 +748,9 @@ class ManyToOneTests(TestCase):
         )
         with self.assertRaisesMessage(ValueError, msg):
             th.child_set.count()
+        # The reverse foreign key manager can be created.
+        self.assertEqual(th.child_set.model, Third)
+
         th.save()
         # Now the model is saved, so we will need to execute a query.
         with self.assertNumQueries(1):