public function orderableColumns()
    {
        if (! $this->isOrderable()) {
            return [];
        }

        $orderable = [];
        for ($i = 0, $c = count($this->request->input('order')); $i < $c; $i++) {
            $order_col = (int) $this->request->input("order.$i.column");
            $order_dir = strtolower($this->request->input("order.$i.dir")) === 'asc' ? 'asc' : 'desc';
            if ($this->isColumnOrderable($order_col)) {
                $orderable[] = ['column' => $order_col, 'direction' => $order_dir];
            }
        }

        return $orderable;
    }