Fixed #24751 -- Fixed HStoreField isnull lookup.

django/contrib/postgres/fields/hstore.py

@@ -78,7 +78,7 @@ class KeyTransform(Transform):
 
     def as_sql(self, compiler, connection):
         lhs, params = compiler.compile(self.lhs)
-        return "%s -> '%s'" % (lhs, self.key_name), params
+        return "(%s -> '%s')" % (lhs, self.key_name), params
 
 
 class KeyTransformFactory(object):

docs/releases/1.8.2.txt

@@ -25,3 +25,5 @@ Bugfixes
   pointing to :class:`~django.db.models.UUIDField` and inheritance on models
   with ``UUIDField`` primary keys work correctly (:ticket:`24698`,
   :ticket:`24712`).
+
+* Fixed ``isnull`` lookup for ``HStoreField`` (:ticket:`24751`).

tests/postgres_tests/test_hstore.py

@@ -114,6 +114,17 @@ class TestQuerying(PostgresSQLTestCase):
             self.objs[:3]
         )
 
+    def test_key_isnull(self):
+        obj = HStoreModel.objects.create(field={'a': None})
+        self.assertSequenceEqual(
+            HStoreModel.objects.filter(field__a__isnull=True),
+            self.objs[2:5] + [obj]
+        )
+        self.assertSequenceEqual(
+            HStoreModel.objects.filter(field__a__isnull=False),
+            self.objs[:2]
+        )
+
 
 class TestSerialization(PostgresSQLTestCase):
     test_data = '[{"fields": {"field": "{\\"a\\": \\"b\\"}"}, "model": "postgres_tests.hstoremodel", "pk": null}]'