Auto update model

I have a model: user (id, username, password, ... key), where "key" is
sha1 hach of username + password.
How can I update key every time when user update own information?

I have a model: user (id, username, password, ... key), where "key" is
sha1 hach of username + password.
How can I update key every time when user update own information?

Look into the "before_save" callback, check to see if values have changed and if so set a new key.

-philip