public function scopeFindByUser($query, $userId)
    {
        if (empty($userId)) return;
        $cart = $query->whereUser($userId)->first();
        if (empty($cart)) {
            $cart = call_user_func( Config::get('shop.cart') . '::create', [
                'user_id' =>  $userId
            ]);
        }
        return $cart;
    }