protected function set_sort_order($sort_field, $sort_order)
    {
        if ($sort_field != null) {
            $this->sort_field = asciiwords($sort_field);
        }
        if ($sort_order != null) {
            $this->sort_order = strtoupper($sort_order) == 'DESC' ? 'DESC' : 'ASC';
        }
    }