Fixed #20820 -- Model inheritance + m2m fixture loading regression

Tests by Tim Graham, report from jeroen.pulles@redslider.net.

django/db/models/fields/related.py

@@ -989,7 +989,16 @@ class ForeignObject(RelatedField):
 
     @staticmethod
     def get_instance_value_for_fields(instance, fields):
-        return tuple([getattr(instance, field.attname) for field in fields])
+        ret = []
+        for field in fields:
+            # 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.
+            if field.primary_key:
+                ret.append(instance.pk)
+            else:
+                ret.append(getattr(instance, field.attname))
+        return tuple(ret)
 
     def get_attname_column(self):
         attname, column = super(ForeignObject, self).get_attname_column()

tests/fixtures_regress/fixtures/special-article.json

+[
+    {
+        "pk": 1,
+        "model": "fixtures_regress.specialarticle",
+        "fields": {
+            "title": "Article Title 1",
+            "channels": []
+        }
+    }
+]

tests/fixtures_regress/models.py

@@ -70,6 +70,11 @@ class Article(models.Model):
         ordering = ('id',)
 
 
+# Subclass of a model with a ManyToManyField for test_ticket_20820
+class SpecialArticle(Article):
+    pass
+
+
 # Models to regression test #11428
 @python_2_unicode_compatible
 class Widget(models.Model):

tests/fixtures_regress/tests.py

@@ -444,6 +444,17 @@ class TestFixtures(TestCase):
         self.assertTrue("No fixture 'this_fixture_doesnt_exist' in" in
             force_text(stdout_output.getvalue()))
 
+    def test_ticket_20820(self):
+        """
+        Regression for ticket #20820 -- loaddata on a model that inherits
+        from a model with a M2M shouldn't blow up.
+        """
+        management.call_command(
+            'loaddata',
+            'special-article.json',
+            verbosity=0,
+        )
+
 
 class NaturalKeyFixtureTests(TestCase):