Pick random result from query.

[eluser]Fantass[/eluser]
Hello, heres my query

Code:

$this->db->having('active', 1);
        $this->db->having('widerange', 2);
        $this->db->having('counter < assigned');
        $query = $this->db->get('anuncios');

Wich return me various results according to the search but i want to pick up a random one
I've tried with Mysql this query

Code:

SELECT * FROM `anuncios`  HAVING `active` = 1 AND `counter` < `assigned` AND `widerange` = 2 ORDER BY RAND() limit 1

What return me exactly what i want.

then i've try adding the line

Code:

$this->db->order_by("id", "RAND");

but nothing ...

Someone could help me?
THANKS.