Validation question

This one I cant seem to wrap my head around…

I have a form that I would like to bring up a :confirm => “are you sure
you want to change your username, doing so may make you foo up.”
but only if a user changes that particular field.

So basically I have:

User name
<%= f.text_field ‘name’ %>

<%= f.text_field ‘email’ %>

<%= submit_tag(“Save changes”) %>

is there something as easy as
validates_change_of :user_name

and if not, what would be a good suggestion an approaching this.

(that would be a nice validates addition if it doesnt exist!)