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