Thinkphp中某个字段值从增或自减函数

/**
 +----------------------
 * 某个字段值自增或自减
 +----------------------
 * @access 
 +----------------------
 * @param string $model 数据表名称
 * @param string $fields 自增或自减字段名称
 * @param integer $value 自增或自减量，默认为1
 * @param boolean $type 布尔值，默认为true自增，为false自减
 +----------------------
 * @return void
 +----------------------
 */
function autoed($id = '',$model = '', $fields = '', $value = '', $type = true) {
	if(!empty($fields)) {
		if(empty($model)) {
			$name = $this->getActionName();	
		} else {
			$name = $model;	
		}
		$model = M($name);
		//默认为主键ID
		$pk = $model->getPk();
		//默认自增自减量为1
		$value = !empty($value) ? $value : 1 ;
		//默认为true时是自增，为false时自减
		$type = $type ? "Inc" : "Dec" ;
		switch($type) {
			case "Inc":
				$model->setInc("$fields", "$pk=$id", $value);
				break;
			case "Dec":	
				$model->setDec("$fields", "$pk=$id", $value);
				$list = $model->field("$fields")->where("$pk=$id")->find();
				if($list[$fields] <= 0){
					$model->where("$pk=$id")->setField("$fields",0);
				}
				break;
		}
	}
}