public function subdistrict($city_id = NULL, $subdistrict_id = NULL){
        $params = array('city' => $city_id);
        if(!is_null($subdistrict_id)) {
            $params['id'] = $subdistrict_id;
        }
        $rest_client = new RESTClient($this->api_key, 'subdistrict', $this->account_type);
        return $rest_client->get($params);
    }