module AbstractController::Callbacks::ClassMethods

* except - The callback should be run for all actions except this action.
* only - The callback should be run only for this action.
==== Options

except: :index, if: -> { true } # the :except option will be ignored.

are used together.
Note that :if has priority over :except in case they

only: :index, if: -> { true } # the :if option will be ignored.

are used together.
Note that :only has priority over :if in case they

:if => proc {|c| c.action_name == "index" }.
The basic idea is that :only => :index gets converted to

+:if+ and +:unless+ options of ActiveSupport::Callbacks.
If +:only+ or +:except+ are used, convert the options into the

def _normalize_callback_options(options)
  _normalize_callback_option(options, :only, :if)
  _normalize_callback_option(options, :except, :unless)
end