Fixed #33414 -- Fixed creating diamond-shaped MTI objects for common ancestor with primary key that has a default.

Co-authored-by: Simon Charette <charette.s@gmail.com>

django/db/models/base.py

@@ -900,10 +900,12 @@ class Model(AltersData, metaclass=ModelBase):
 
     save_base.alters_data = True
 
-    def _save_parents(self, cls, using, update_fields):
+    def _save_parents(self, cls, using, update_fields, updated_parents=None):
         """Save all the parents of cls using values from self."""
         meta = cls._meta
         inserted = False
+        if updated_parents is None:
+            updated_parents = {}
         for parent, field in meta.parents.items():
             # Make sure the link fields are synced between parent and self.
             if (
@@ -912,16 +914,23 @@ class Model(AltersData, metaclass=ModelBase):
                 and getattr(self, field.attname) is not None
             ):
                 setattr(self, parent._meta.pk.attname, getattr(self, field.attname))
-            parent_inserted = self._save_parents(
-                cls=parent, using=using, update_fields=update_fields
-            )
-            updated = self._save_table(
-                cls=parent,
-                using=using,
-                update_fields=update_fields,
-                force_insert=parent_inserted,
-            )
-            if not updated:
+            if (parent_updated := updated_parents.get(parent)) is None:
+                parent_inserted = self._save_parents(
+                    cls=parent,
+                    using=using,
+                    update_fields=update_fields,
+                    updated_parents=updated_parents,
+                )
+                updated = self._save_table(
+                    cls=parent,
+                    using=using,
+                    update_fields=update_fields,
+                    force_insert=parent_inserted,
+                )
+                if not updated:
+                    inserted = True
+                updated_parents[parent] = updated
+            elif not parent_updated:
                 inserted = True
             # Set the parent's PK value to self.
             if field:

tests/model_inheritance/models.py

@@ -186,3 +186,23 @@ class Child(Parent):
 
 class GrandChild(Child):
     pass
+
+
+class CommonAncestor(models.Model):
+    id = models.IntegerField(primary_key=True, default=1)
+
+
+class FirstParent(CommonAncestor):
+    first_ancestor = models.OneToOneField(
+        CommonAncestor, models.CASCADE, primary_key=True, parent_link=True
+    )
+
+
+class SecondParent(CommonAncestor):
+    second_ancestor = models.OneToOneField(
+        CommonAncestor, models.CASCADE, primary_key=True, parent_link=True
+    )
+
+
+class CommonChild(FirstParent, SecondParent):
+    pass

tests/model_inheritance/tests.py

@@ -9,6 +9,7 @@ from django.test.utils import CaptureQueriesContext, isolate_apps
 from .models import (
     Base,
     Chef,
+    CommonChild,
     CommonInfo,
     CustomSupplier,
     GrandChild,
@@ -149,6 +150,14 @@ class ModelInheritanceTests(TestCase):
         # accidentally found).
         self.assertSequenceEqual(s.titles.all(), [])
 
+    def test_create_diamond_mti_default_pk(self):
+        # 1 INSERT for each base.
+        with self.assertNumQueries(4):
+            common_child = CommonChild.objects.create()
+        # 3 SELECTs for the parents, 1 UPDATE for the child.
+        with self.assertNumQueries(4):
+            common_child.save()
+
     def test_update_parent_filtering(self):
         """
         Updating a field of a model subclass doesn't issue an UPDATE