fix test_contains_subquery

Author: timgraham

django_mongodb_backend/features.py

@@ -80,9 +80,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "auth_tests.test_views.LoginTest.test_login_session_without_hash_session_key",
         # GenericRelation.value_to_string() assumes integer pk.
         "contenttypes_tests.test_fields.GenericRelationTests.test_value_to_string",
-        # contains with Exists() doesn't work:
-        # https://github.com/mongodb-labs/django-mongodb/issues/204
-        "model_fields_.test_arrayfield.QueryingTests.test_contains_subquery",
         # overlap with values() returns no results:
         # https://github.com/mongodb-labs/django-mongodb/issues/209
         "model_fields_.test_arrayfield.QueryingTests.test_overlap_values",

django_mongodb_backend/fields/array.py

@@ -244,7 +244,13 @@ class ArrayContains(ArrayRHSMixin, FieldGetDbPrepValueMixin, Lookup):
     def as_mql(self, compiler, connection):
         lhs_mql = process_lhs(self, compiler, connection)
         value = process_rhs(self, compiler, connection)
-        return {"$and": [{"$ne": [lhs_mql, None]}, {"$setIsSubset": [value, lhs_mql]}]}
+        return {
+            "$and": [
+                {"$ne": [lhs_mql, None]},
+                {"$ne": [value, None]},
+                {"$setIsSubset": [value, lhs_mql]},
+            ]
+        }
 
 
 @ArrayField.register_lookup