Fixed #25972 -- Restored support for the isnull lookup with ForeignObject.

django/db/models/fields/related.py

@@ -25,7 +25,7 @@ from .related_descriptors import (
 )
 from .related_lookups import (
     RelatedExact, RelatedGreaterThan, RelatedGreaterThanOrEqual, RelatedIn,
-    RelatedLessThan, RelatedLessThanOrEqual,
+    RelatedIsNull, RelatedLessThan, RelatedLessThanOrEqual,
 )
 from .reverse_related import (
     ForeignObjectRel, ManyToManyRel, ManyToOneRel, OneToOneRel,
@@ -672,9 +672,10 @@ class ForeignObject(RelatedField):
             return RelatedLessThan
         elif lookup_name == 'lte':
             return RelatedLessThanOrEqual
-        elif lookup_name != 'isnull':
+        elif lookup_name == 'isnull':
+            return RelatedIsNull
+        else:
             raise TypeError('Related Field got invalid lookup: %s' % lookup_name)
-        return super(ForeignObject, self).get_lookup(lookup_name)
 
     def get_transform(self, *args, **kwargs):
         raise NotImplementedError('Relational fields do not support transforms.')

django/db/models/fields/related_lookups.py

 from django.db.models.lookups import (
-    Exact, GreaterThan, GreaterThanOrEqual, In, LessThan, LessThanOrEqual,
+    Exact, GreaterThan, GreaterThanOrEqual, In, IsNull, LessThan,
+    LessThanOrEqual,
 )
 
 
@@ -131,3 +132,7 @@ class RelatedGreaterThanOrEqual(RelatedLookupMixin, GreaterThanOrEqual):
 
 class RelatedLessThanOrEqual(RelatedLookupMixin, LessThanOrEqual):
     pass
+
+
+class RelatedIsNull(RelatedLookupMixin, IsNull):
+    pass

docs/releases/1.9.1.txt

@@ -64,3 +64,6 @@ Bugfixes
 
 * Restored the functionality of the admin's ``list_editable`` add and change
   buttons (:ticket:`25903`).
+
+* Fixed ``isnull`` query lookup for ``ForeignObject``
+  (:ticket:`25972`).

tests/foreign_object/models/person.py

@@ -56,19 +56,19 @@ class Membership(models.Model):
     date_joined = models.DateTimeField(default=datetime.datetime.now)
     invite_reason = models.CharField(max_length=64, null=True)
     person_id = models.IntegerField()
-    group_id = models.IntegerField()
+    group_id = models.IntegerField(blank=True, null=True)
 
     # Relation Fields
     person = models.ForeignObject(
         Person,
-        from_fields=['membership_country', 'person_id'],
-        to_fields=['person_country_id', 'id'],
+        from_fields=['person_id', 'membership_country'],
+        to_fields=['id', 'person_country_id'],
         on_delete=models.CASCADE,
     )
     group = models.ForeignObject(
         Group,
-        from_fields=['membership_country', 'group_id'],
-        to_fields=['group_country', 'id'],
+        from_fields=['group_id', 'membership_country'],
+        to_fields=['id', 'group_country'],
         on_delete=models.CASCADE,
     )
 
@@ -76,7 +76,8 @@ class Membership(models.Model):
         ordering = ('date_joined', 'invite_reason')
 
     def __str__(self):
-        return "%s is a member of %s" % (self.person.name, self.group.name)
+        group_name = self.group.name if self.group_id else 'NULL'
+        return "%s is a member of %s" % (self.person.name, group_name)
 
 
 class Friendship(models.Model):

tests/foreign_object/tests.py

@@ -395,6 +395,18 @@ class MultiColumnFKTests(TestCase):
         objs = [Person(name="abcd_%s" % i, person_country=self.usa) for i in range(0, 5)]
         Person.objects.bulk_create(objs, 10)
 
+    def test_isnull_lookup(self):
+        Membership.objects.create(membership_country=self.usa, person=self.bob, group_id=None)
+        Membership.objects.create(membership_country=self.usa, person=self.bob, group=self.cia)
+        self.assertQuerysetEqual(
+            Membership.objects.filter(group__isnull=True),
+            ['<Membership: Bob is a member of NULL>']
+        )
+        self.assertQuerysetEqual(
+            Membership.objects.filter(group__isnull=False),
+            ['<Membership: Bob is a member of CIA>']
+        )
+
 
 class TestModelCheckTests(SimpleTestCase):