public function init_expander($field)
    {
        // set column style
        $this->columns[$field]['thparam'] .= ' style="width:40px; text-align:center"';

        // set column refid - referenced model table for example
        if (!isset($this->columns[$field]['refid'])) {
            if ($this->model) {
                $refid = $this->model->table;
            } elseif ($this->dq) {
                $refid = $this->dq->args['table'];
            } else {
                $refid = preg_replace('/.*_/', '', $this->app->page);
            }

            $this->columns[$field]['refid'] = $refid;
        }

        // initialize button widget on page load
        $class = $this->name.'_'.$field.'_expander';
        $this->js(true)->find('.'.$class)->button();

        // initialize expander
        $this->js(true)
            ->_selector('.'.$class)
            ->_load('ui.atk4_expander')
            ->atk4_expander();
    }