public function delete($id)
 {
     $this->db->start_transaction();
     # delete city related by state
     $city = new CityModel();
     $city->delete_by_state($id);
     # remove the state itself
     $condition = array(StateModel::$primary_key => $id);
     $this->db->delete(StateModel::$table_name, $condition);
     $this->db->complete_transaction();
     return $this->db->transaction_status();
 }