[1.6.x] Fixed #21472 -- Fixed inline formsets display when parent pk is 0

Thanks agale031176@gmail.com for the report.
Backport of fafb6cf049 from master.

django/forms/models.py

@@ -831,7 +831,7 @@ class BaseInlineFormSet(BaseModelFormSet):
         self.save_as_new = save_as_new
         if queryset is None:
             queryset = self.model._default_manager
-        if self.instance.pk:
+        if self.instance.pk is not None:
             qs = queryset.filter(**{self.fk.name: self.instance})
         else:
             qs = queryset.none()

docs/releases/1.6.1.txt

@@ -26,3 +26,5 @@ Bug fixes
   for the non-breaking space was returned verbatim (#21415).
 * Fixed :djadmin:`loaddata` error when fixture file name contained any dots
   non related to file extensions (#21457).
+* Fixed display of inline instances in formsets when parent has 0 for primary
+  key (#21472).

tests/inline_formsets/tests.py

@@ -1,7 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 from django.forms.models import inlineformset_factory
-from django.test import TestCase
+from django.test import TestCase, skipUnlessDBFeature
 from django.utils import six
 
 from .models import Poet, Poem, School, Parent, Child
@@ -157,3 +157,12 @@ class InlineFormsetFactoryTest(TestCase):
         inlineformset_factory(
             Parent, Child, exclude=('school',), fk_name='mother'
         )
+
+    @skipUnlessDBFeature('allows_primary_key_0')
+    def test_zero_primary_key(self):
+        # Regression test for #21472
+        poet = Poet.objects.create(id=0, name='test')
+        poem = poet.poem_set.create(name='test poem')
+        PoemFormSet = inlineformset_factory(Poet, Poem, fields="__all__", extra=0)
+        formset = PoemFormSet(None, instance=poet)
+        self.assertEqual(len(formset.forms), 1)