Define ObjectIdAutoField rel_db_type.

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