In the previous version, Kohana 2.1.2, I used before. I can use findby*() method like this:
$user = new User_Model(); $user->find_by_username('tzangms');
But for now, in Kohana 2.2, we should use where() instead, if we want to find by username, not by id.
$user = new User_Model() $user->where('username', 'tzangms')->find();
damn, findby* is so intuitive.