Fixed #24266 -- Changed get_parent_list to return a list ordered by MRO.

Thanks to Aron Podrigal for the initial patch and Tim for the review.

django/db/models/options.py

@@ -12,7 +12,7 @@ from django.db.models.fields.related import ManyToManyField
 from django.db.models.fields import AutoField
 from django.db.models.fields.proxy import OrderWrt
 from django.utils import six
-from django.utils.datastructures import ImmutableList
+from django.utils.datastructures import ImmutableList, OrderedSet
 from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.encoding import force_text, smart_text, python_2_unicode_compatible
 from django.utils.functional import cached_property
@@ -634,14 +634,14 @@ class Options(object):
 
     def get_parent_list(self):
         """
-        Returns a list of all the ancestor of this model as a list. Useful for
-        determining if something is an ancestor, regardless of lineage.
+        Returns all the ancestors of this model as a list ordered by MRO.
+        Useful for determining if something is an ancestor, regardless of lineage.
         """
-        result = set()
+        result = OrderedSet(self.parents)
         for parent in self.parents:
-            result.add(parent)
-            result.update(parent._meta.get_parent_list())
-        return result
+            for ancestor in parent._meta.get_parent_list():
+                result.add(ancestor)
+        return list(result)
 
     def get_ancestor_link(self, ancestor):
         """

tests/model_meta/models.py

@@ -118,3 +118,20 @@ class Relating(models.Model):
     # ManyToManyField to Person
     people = models.ManyToManyField(Person, related_name='relating_people')
     people_hidden = models.ManyToManyField(Person, related_name='+')
+
+
+# ParentListTests models
+class CommonAncestor(models.Model):
+    pass
+
+
+class FirstParent(CommonAncestor):
+    first_ancestor = models.OneToOneField(CommonAncestor, primary_key=True, parent_link=True)
+
+
+class SecondParent(CommonAncestor):
+    second_ancestor = models.OneToOneField(CommonAncestor, primary_key=True, parent_link=True)
+
+
+class Child(FirstParent, SecondParent):
+    pass

tests/model_meta/tests.py

@@ -5,7 +5,10 @@ from django.db.models.fields import related, CharField, Field
 from django.db.models.options import IMMUTABLE_WARNING, EMPTY_RELATION_TREE
 from django.test import TestCase
 
-from .models import Relation, AbstractPerson, BasePerson, Person, ProxyPerson, Relating
+from .models import (
+    Relation, AbstractPerson, BasePerson, Person, ProxyPerson, Relating,
+    CommonAncestor, FirstParent, SecondParent, Child
+)
 from .results import TEST_RESULTS
 
 
@@ -245,3 +248,11 @@ class RelationTreeTests(TestCase):
             ])
         )
         self.assertEqual([field.related_query_name() for field in AbstractPerson._meta._relation_tree], [])
+
+
+class ParentListTests(TestCase):
+    def test_get_parent_list(self):
+        self.assertEqual(CommonAncestor._meta.get_parent_list(), [])
+        self.assertEqual(FirstParent._meta.get_parent_list(), [CommonAncestor])
+        self.assertEqual(SecondParent._meta.get_parent_list(), [CommonAncestor])
+        self.assertEqual(Child._meta.get_parent_list(), [FirstParent, SecondParent, CommonAncestor])