Error when using db library (set to autoload)

[eluser]Ren83[/eluser]

Code:

<?php

class Staff_m extends CI_Model
{
        private $serial;
        public function __construct() {
                  parent::__construct();
        }

        public function get_staff_data($serial){

                  $this->serial = $serial;
                  $result = $this->db->query('SELECT * FROM staff WHERE serial ='.$serial);

        }
}
?>