protected function _mapModelToMongoModel(array $model)
 {
     $model = parent::_mapModelToMongoModel($model);
     if (isset($model['expireDatetime']) && !$model['expireDatetime'] instanceof \MongoDate) {
         if (is_string($model['expireDatetime'])) {
             $model['expireDatetime'] = strtotime($model['expireDatetime']);
         }
         $model['expireDatetime'] = new \MongoDate($model['expireDatetime']);
     }
     return $model;
 }