remove unnecessary $convert in ObjectIdAutoField $lookup queries

Author: WaVEV

django_mongodb/fields/auto.py

@@ -34,6 +34,9 @@ def get_prep_value(self, value):
     def db_type(self, connection):
         return "objectId"
 
+    def rel_db_type(self, connection):
+        return "objectId"
+
     def to_python(self, value):
         if value is None or isinstance(value, int):
             return value

tests/queries_/test_mql.py

@@ -18,9 +18,9 @@ def test_join(self):
             query,
             "db.queries__book.aggregate(["
             "{'$lookup': {'from': 'queries__author', "
-            "'let': {'parent__field__0': {'$convert': {'input': '$author_id', 'to': 'string'}}}, "
-            "'pipeline': [{'$match': {'$expr': {'$and': [{'$eq': ['$$parent__field__0', "
-            "{'$convert': {'input': '$_id', 'to': 'string'}}]}]}}}], 'as': 'queries__author'}}, "
+            "'let': {'parent__field__0': '$author_id'}, "
+            "'pipeline': [{'$match': {'$expr': "
+            "{'$and': [{'$eq': ['$$parent__field__0', '$_id']}]}}}], 'as': 'queries__author'}}, "
             "{'$unwind': '$queries__author'}, "
             "{'$match': {'$expr': {'$eq': ['$queries__author.name', 'Bob']}}}])",
         )