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

@@ -1,5 +1,6 @@
 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