How do you create a callback with an if statement for multiple changed fields in Ruby on Rails?

Question

I would like to create a before_save callback that only gets run if there have been changes to any (but not necessarily all) of the three fields (street, city,

Answer 1

You can do it in one line using a Proc:

class User
  before_save :run_test_method, :if => Proc.new { |u| u.street_changed? || u.city_changed? || u.state_changed? }

  ...
end