protected function getSortableClass($column_key)
    {
        $classes = NULL;

        if ($this->getColumnOption($column_key, self::COLUMN_OPTION_SORTABLE))
        {
            $classes .= 'sorting ';
            if (array_key_exists($column_key, $this->columnOrders))
            {
                $order_method = $this->columnOrders[$column_key];
                switch ($order_method)
                {
                    case 'desc':
                        $classes .= 'sorting_asc ';
                        break;
                    default:
                        $classes .= 'sorting_desc ';
                }
            }
        }

        return $classes;
    }