$this->db->distinct() help needed

[eluser]Computerzworld[/eluser]
Hello. I want to use $this->db->distinct on a particular column lets say trainId. How can I use it? I have passed trainId as parameter like this. $this->db->distinct('trainId') but it didn't returned me distinct result. Here is the exact code which I have written.

Code:

$this->db->distinct('trainId');
$this->db->where_in('stationId', $stationIds);
$query = $this->db->get("tbl_tt_schedule");

Please help me. Thanks in advance.