Yii2.0 无法更新记录？ [ 2.0 版本 ]

控制器action:

public function actionBasic() {
    $model = new BasicForm();

    if ($model->load(Yii::$app->request->post()) && $model->validate()) {
        $result = $model->basicSave($this->_userid);
        die('保存成功！');

    } else {
        echo 'find';
        $model = Seller::findOne($this->_userid);

        return $this->render('basic', [
            'model' => $model,
        ]);
    }
}

Model:

<?php

namespace frontend\models;

use Yii;
use yii\base\Model;

use frontend\models\Seller;

/**
 * ContactForm is the model behind the contact form.
 */
class BasicForm extends Model
{
    public $shop_name;
    public $tel;
    public $sign;
    public $realname;

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['shop_name', 'tel', 'sign', 'realname'], 'required'],
        ];
    }

    /**
     * @inheritdoc
     */
    public function attributeLabels()
    {
        /*return [
            'verifyCode' => 'Verification Code',
        ];*/
    }

    public function basicSave($userid) {

        $user = Seller::findOne($userid);
        $user->shop_name = $this->shop_name;
        $user->tel = $this->tel;
        $user->sign = $this->sign;
        $user->realname = $this->realname;

        //$user->generateAuthKey();
        if ($user->save()) {
            return $user;
        }

        return null;
    }

}

在post的时候， 这句一直为false，哪里问题？

if ($model->load(Yii::$app->request->post()) && $model->validate())