Fixed #20946 -- model inheritance + m2m failure

Cleaned up the internal implementation of m2m fields by removing
related.py _get_fk_val(). The _get_fk_val() was doing the wrong thing
if asked for the foreign key value on foreign key to parent model's
primary key when child model had different primary key field.

django/db/models/fields/related.py

@@ -503,8 +503,6 @@ def create_many_related_manager(superclass, rel):
             self.through = through
             self.prefetch_cache_name = prefetch_cache_name
             self.related_val = source_field.get_foreign_related_value(instance)
-            # Used for single column related auto created models
-            self._fk_val = self.related_val[0]
             if None in self.related_val:
                 raise ValueError('"%r" needs to have a value for field "%s" before '
                                  'this many-to-many relationship can be used.' %
@@ -517,18 +515,6 @@ def create_many_related_manager(superclass, rel):
                                  "a many-to-many relationship can be used." %
                                  instance.__class__.__name__)
 
-        def _get_fk_val(self, obj, field_name):
-            """
-            Returns the correct value for this relationship's foreign key. This
-            might be something else than pk value when to_field is used.
-            """
-            fk = self.through._meta.get_field(field_name)
-            if fk.rel.field_name and fk.rel.field_name != fk.rel.to._meta.pk.attname:
-                attname = fk.rel.get_related_field().get_attname()
-                return fk.get_prep_lookup('exact', getattr(obj, attname))
-            else:
-                return obj.pk
-
         def get_queryset(self):
             try:
                 return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
@@ -626,11 +612,12 @@ def create_many_related_manager(superclass, rel):
                         if not router.allow_relation(obj, self.instance):
                             raise ValueError('Cannot add "%r": instance is on database "%s", value is on database "%s"' %
                                                (obj, self.instance._state.db, obj._state.db))
-                        fk_val = self._get_fk_val(obj, target_field_name)
+                        fk_val = self.through._meta.get_field(
+                            target_field_name).get_foreign_related_value(obj)[0]
                         if fk_val is None:
                             raise ValueError('Cannot add "%r": the value for field "%s" is None' %
                                              (obj, target_field_name))
-                        new_ids.add(self._get_fk_val(obj, target_field_name))
+                        new_ids.add(fk_val)
                     elif isinstance(obj, Model):
                         raise TypeError("'%s' instance expected, got %r" % (self.model._meta.object_name, obj))
                     else:
@@ -638,7 +625,7 @@ def create_many_related_manager(superclass, rel):
                 db = router.db_for_write(self.through, instance=self.instance)
                 vals = self.through._default_manager.using(db).values_list(target_field_name, flat=True)
                 vals = vals.filter(**{
-                    source_field_name: self._fk_val,
+                    source_field_name: self.related_val[0],
                     '%s__in' % target_field_name: new_ids,
                 })
                 new_ids = new_ids - set(vals)
@@ -652,7 +639,7 @@ def create_many_related_manager(superclass, rel):
                 # Add the ones that aren't there already
                 self.through._default_manager.using(db).bulk_create([
                     self.through(**{
-                        '%s_id' % source_field_name: self._fk_val,
+                        '%s_id' % source_field_name: self.related_val[0],
                         '%s_id' % target_field_name: obj_id,
                     })
                     for obj_id in new_ids
@@ -676,7 +663,9 @@ def create_many_related_manager(superclass, rel):
                 old_ids = set()
                 for obj in objs:
                     if isinstance(obj, self.model):
-                        old_ids.add(self._get_fk_val(obj, target_field_name))
+                        fk_val = self.through._meta.get_field(
+                            target_field_name).get_foreign_related_value(obj)[0]
+                        old_ids.add(fk_val)
                     else:
                         old_ids.add(obj)
                 # Work out what DB we're operating on
@@ -690,7 +679,7 @@ def create_many_related_manager(superclass, rel):
                         model=self.model, pk_set=old_ids, using=db)
                 # Remove the specified objects from the join table
                 self.through._default_manager.using(db).filter(**{
-                    source_field_name: self._fk_val,
+                    source_field_name: self.related_val[0],
                     '%s__in' % target_field_name: old_ids
                 }).delete()
                 if self.reverse or source_field_name == self.source_field_name:
@@ -994,10 +983,13 @@ class ForeignObject(RelatedField):
             # Gotcha: in some cases (like fixture loading) a model can have
             # different values in parent_ptr_id and parent's id. So, use
             # instance.pk (that is, parent_ptr_id) when asked for instance.id.
+            opts = instance._meta
             if field.primary_key:
-                ret.append(instance.pk)
-            else:
-                ret.append(getattr(instance, field.attname))
+                possible_parent_link = opts.get_ancestor_link(field.model)
+                if not possible_parent_link or possible_parent_link.primary_key:
+                    ret.append(instance.pk)
+                    continue
+            ret.append(getattr(instance, field.attname))
         return tuple(ret)
 
     def get_attname_column(self):

tests/model_inheritance/models.py

@@ -162,3 +162,9 @@ class Mixin(object):
 
 class MixinModel(models.Model, Mixin):
     pass
+
+class Base(models.Model):
+    titles = models.ManyToManyField(Title)
+
+class SubBase(Base):
+    sub_id = models.IntegerField(primary_key=True)

tests/model_inheritance/tests.py

@@ -10,7 +10,8 @@ from django.utils import six
 
 from .models import (
     Chef, CommonInfo, ItalianRestaurant, ParkingLot, Place, Post,
-    Restaurant, Student, StudentWorker, Supplier, Worker, MixinModel)
+    Restaurant, Student, StudentWorker, Supplier, Worker, MixinModel,
+    Title, Base, SubBase)
 
 
 class ModelInheritanceTests(TestCase):
@@ -357,3 +358,16 @@ class ModelInheritanceTests(TestCase):
             [Place.objects.get(pk=s.pk)],
             lambda x: x
         )
+
+    def test_custompk_m2m(self):
+        b = Base.objects.create()
+        b.titles.add(Title.objects.create(title="foof"))
+        s = SubBase.objects.create(sub_id=b.id)
+        b = Base.objects.get(pk=s.id)
+        self.assertNotEqual(b.pk, s.pk)
+        # Low-level test for related_val
+        self.assertEqual(s.titles.related_val, (s.id,))
+        # Higher level test for correct query values (title foof not
+        # accidentally found).
+        self.assertQuerysetEqual(
+            s.titles.all(), [])