CodeIgniter如何对连接记录进行排序

I have following query in CodeIgniter:

$user = $this->db
    ->select('users.*, login_logs.ip_address, login_logs.date, login_logs.userid')
    ->from('db.users')
    ->join('db.login_logs', 'users.userid = login_logs.userid')
    ->group_by('users.userid')
    ->get()
    ->result_array();

It works very good but I have to select last record for each user( in login_logs table) but now I'm getting first record. I have tried :

->order_by('login_logs.idlogin_logs', 'DESC')

but it didn't work.

I have no idea how to do it. Is there any easy way to reach it?

@edit

I have triend ASC/DESC. No changes.

Present query:

 SELECT `users`.*, `login_logs`.`ip_address`, `login_logs`.`date`, `login_logs`.`userid`
FROM `db`.`users`
JOIN `db`.`login_logs` ON `users`.`userid` = `login_logs`.`userid`
GROUP BY `users`.`userid`