not sure why a Proc would be used here - not getting something simple

Question

I understand the concept of a proc but sometime I see code like this (take from rails guide for validation http://guides.rubyonrails.org/active_record_validations_callbacks.html#using-if-and-unless-with-a-proc):

class Order < ActiveRecord::Base
  before_save :normalize_card_number,
    :if => Proc.new { |order| order.paid_with_card? }
end

it seems this could be more simply written as:

class Order < ActiveRecord::Base
  before_save :normalize_card_number, :if => :paid_with_card? 
end

What am I not getting about the advantage of using a Proc here?

thx in advance

Answer 1

In simple cases they are equivalent, but procs allow far more versatility without requiring a method to be defined simply for a validation if check.

Imagine this:

before_save :nuke, :if => Proc.new { |model| !model.nuked? && model.nukable? && model.region.nukable? }

You can always write that check in an instance method and reference it with a symbol, but for cases where the specific logic is only in the :if, it is valid to keep it in a proc.