Question about ORM(Update)

[eluser]InterMedio[/eluser]
Hello everybody.
I wrote model t_activity.php

Code:

class T_activity extends CI_Model {

function last_act($user_id, $current) {
  $this->db->query("UPDATE sessions
        SET last_activity='$current'
        WHERE SUBSTRING_INDEX(SUBSTRING_INDEX(user_data, '\"', 4) , '\"', -1)='$user_id'
       ");
}

}

But when I put this method into controller:

Code:

$current_ = time();
$this->t_activity->last_act($current_, $user_id);

this doesn't working.
This working, when I put this sql request in controller.
But I want store this sql query in model.
Anybody can tell me, where can be error or how correct write this request in ORM syntax?
Thanks all.