/**
  * Finds the AClass model based on its primary key value.
  * If the model is not found, a 404 HTTP exception will be thrown.
  * @param integer $id
  * @return AClass the loaded model
  * @throws NotFoundHttpException if the model cannot be found
  */
 protected function findModel($id)
 {
     if (($model = AClass::findOne($id)) !== null) {
         return $model;
     } else {
         throw new NotFoundHttpException('The requested page does not exist.');
     }
 }