Restrict callback to fields

please can some kind soul point me to the right track.

how can I express:

“do something when any field of this class is updated except field X”

I need to do anything that would behave like

after_update :do_something #unless the updated field is field X

that is: after_update is called every time a set_ method is called,
except set_x

Is that at all possible?

Isabelle