fix GenericReference iterable query (i.e. __in)

mongoengine/queryset/transform.py

@@ -129,13 +129,15 @@ def query(_doc_cls=None, **kwargs):
 
             singular_ops = [None, "ne", "gt", "gte", "lt", "lte", "not"]
             singular_ops += STRING_OPERATORS
+            is_iterable = False
             if op in singular_ops:
                 value = field.prepare_query_value(op, value)
 
                 if isinstance(field, CachedReferenceField) and value:
                     value = value["_id"]
 
             elif op in ("in", "nin", "all", "near") and not isinstance(value, dict):
+                is_iterable = True
                 # Raise an error if the in/nin/all/near param is not iterable.
                 value = _prepare_query_for_iterable(field, op, value)
 
@@ -144,10 +146,24 @@ def query(_doc_cls=None, **kwargs):
             # * If the value is a DBRef, the key should be "field_name._ref".
             # * If the value is an ObjectId, the key should be "field_name._ref.$id".
             if isinstance(field, GenericReferenceField):
-                if isinstance(value, DBRef):
+                if isinstance(value, DBRef) or (
+                    is_iterable and all(isinstance(v, DBRef) for v in value)
+                ):
                     parts[-1] += "._ref"
-                elif isinstance(value, ObjectId):
+                elif isinstance(value, ObjectId) or (
+                    is_iterable and all(isinstance(v, ObjectId) for v in value)
+                ):
                     parts[-1] += "._ref.$id"
+                elif (
+                    is_iterable
+                    and any(isinstance(v, DBRef) for v in value)
+                    and any(isinstance(v, ObjectId) for v in value)
+                ):
+                    raise ValueError(
+                        "The `in`, `nin`, `all`, or `near`-operators cannot "
+                        "be applied to mixed queries of DBRef/ObjectId/%s"
+                        % _doc_cls.__name__
+                    )
 
         # if op and op not in COMPARISON_OPERATORS:
         if op:

tests/queryset/test_transform.py

@@ -396,6 +396,34 @@ class Shop(Document):
 
         Shop.drop_collection()
 
+    def test_transform_generic_reference_field(self):
+        class Object(Document):
+            field = GenericReferenceField()
+
+        Object.drop_collection()
+        objects = Object.objects.insert([Object() for _ in range(8)])
+        # singular queries
+        assert transform.query(Object, field=objects[0].pk) == {
+            "field._ref.$id": objects[0].pk
+        }
+        assert transform.query(Object, field=objects[1].to_dbref()) == {
+            "field._ref": objects[1].to_dbref()
+        }
+
+        # iterable queries
+        assert transform.query(Object, field__in=[objects[2].pk, objects[3].pk]) == {
+            "field._ref.$id": {"$in": [objects[2].pk, objects[3].pk]}
+        }
+        assert transform.query(
+            Object, field__in=[objects[4].to_dbref(), objects[5].to_dbref()]
+        ) == {"field._ref": {"$in": [objects[4].to_dbref(), objects[5].to_dbref()]}}
+
+        # invalid query
+        with pytest.raises(match="cannot be applied to mixed queries"):
+            transform.query(Object, field__in=[objects[6].pk, objects[7].to_dbref()])
+
+        Object.drop_collection()
+
 
 if __name__ == "__main__":
     unittest.main()