fix ArrayField contains lookup crash with Exists

fixes mongodb#204

Author: WaVEV

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