public function delete($shift_id)
 {
     $this->common->authenticate();
     $message = $this->common->get_message('delete_shift', array('delete_success', 'delete_failure'));
     if (Shifts_model::delete_shift($shift_id)) {
         $data = array('status' => 'success', 'message' => $message['delete_success']);
     } else {
         $data = array('status' => 'failure', 'message' => $message['delete_failure']);
     }
     echo json_encode($data);
 }