last_login type field

Hi,

In my user authentication I'd like to be able to register when a user
last logged on to the system. I currently have an update_usage method
in my model that sets a last_login field to Time.now() on successful
login. The table also includes the updated_on field.

Is there a way of 'touching' a record so that the updated_on field is
set even though no other record fields are updated? At the moment
there is some unnecessary duplication.

Thanks,
Tobe