protected function getSortableLinkStart($column_key)
    {
        if ($this->getColumnOption($column_key, self::COLUMN_OPTION_SORTABLE, false))
        {
            $sortable_url = $this->createUrl(array(
                "column_orders[{$column_key}]" => (isset($this->columnOrders[$column_key]) && $this->columnOrders[$column_key] == 'asc') ? 'desc' : 'asc',
                'single_order' => $column_key
            ));

            echo '<a href="' . $sortable_url . '">';
        }
    }