yii2.0自定义的验证规则不起作用 [ 新手入门 ]

以下为model中的代码，自定义的validateShopPrice方法不起作用，请问是什么原因啊

namespace common\models;
use Yii;
use yii\web\UploadedFile;
use common\tools\UploadFile;
class Goods extends \core\base\BaseActiveRecord
{
    public $attribute;
    public static function tableName()
    {
        return '{{%goods}}';
    }
    public function beforeSave($insert){
        $uploadFile = UploadFile::uploadFiles("Goods[middle_img]","goodsimg");
        if($uploadFile!=null){
            $this->middle_img = $uploadFile['geturl'].$uploadFile['new_name'];
            echo $this->middle_img;
        }
        return parent::beforeSave($insert);
    }  
    public function rules()
    {
        return [
            [['goods_name', ], 'required','message'=>'商品名称不能为空'],
            ['shop_price','validateShopPrice','skipOnError' => false],
        ];
    }
    public function validateShopPrice($attribute, $params)
    {
      if (($this->$attribute)<1000){
           $this->addError($attribute,'价格应大于1000');
           return false;
      }
      return true;
    }