Refetches a list of fields from the DB

def refetch_fields(self, missing_fields):
        """ Refetches a list of fields from the DB """
        db_fields = self.mongokat_collection.find_one({"_id": self["_id"]}, fields={k: 1 for k in missing_fields})

        self._fetched_fields += tuple(missing_fields)

        if not db_fields:
            return

        for k, v in db_fields.items():
            self[k] = v