public static function findPreviousElement(Model $model, $type=null)
	{
		if($type === null) {
			$type = $model->getType();
		}

		$column = array('pid=?', 'ptable=?', 'type=?', 'sorting<?');
		$values = array($model->pid, $model->ptable, $model->getTypeName($type), $model->sorting);

		return \ContentModel::findOneBy($column, $values);
	}