How to compare two columns in a JOIN with CI

[eluser]tochie[/eluser]
hello brethren,

I want to do something like this in CI

Code:

JOIN messages_sent ON messages_sent.msg_key=$msg_key AND messages_sent.msg_from=$mem_code

I tried doing it this way but it didnt work correctly..

Code:

$this->db->join('messages_sent', 'messages_sent.msg_key='.$msg_key);
$this->db->join('messages_sent', 'messages_sent.msg_from='.$mem_code);

Please any suggests as to the correct way to compare two columns in one query...Thanks