Model, controller or view - where to normalize data?

I have a login screen and I wish to normalize the login name for the
purposes of INSERT and SELECT. In the model I have:

def username=(name)
# keycase is a local extension of class String.
write_attribute(:username, name.keycase)

The attribute username of an new user is indeed stored as a normalized
string. However, when logging on and I input the username in capital
letters then the select fails, because the param value of username is
not normalized. The question is, where do I perform this manipulation
for the login? In the controller? In the view? Or is there a way to
accomplish this in the model?