public function order($column)
    {
        if (!in_array($column, $this->allowedOrders)) {
            return null;
        }
        
        if (isset($this->order[$column])) {
            $order = strtolower($this->order[$column]) == self::ORDER_ASC ? self::ORDER_DESC : self::ORDER_ASC;
        } else {
            $order = self::ORDER_ASC;
        }

        return $this->getUrl(['order-' . $column => $order]);
    }