def METHOD_NAME(self, obj):
    filter_string_dict = {"sourceSystemIds": [], "tags": []}
    filter_keys = obj.filter.keys()
    if "sourceSystemIds" in filter_keys:
        filter_string_dict["sourceSystemIds"] = obj.filter["sourceSystemIds"]
    if "tags" in filter_keys:
        filter_string_dict["tags"] = obj.filter["tags"]
    obj.filter_string = json.dumps(filter_string_dict)
    return super().METHOD_NAME(obj)