/**
* 根据 category_id 维护sub_ids字段数据
* @param $category_id int
* @return string
*/
public function get_sub_ids_by_category_id($category_id)
{
$result = array();
$category = $this->get($category_id);
$categories = $this->load();
$result = tree::get_childs($categories, $category_id);
$str = implode(',', $result);
return $str;
}