Metaprogramming Ruby 2

The Rise of alias_method_chain

In ​The Include-and-Extend Trick​, I showed you a snippet of code from an old version of Rails…minus a few interesting lines. Here is the same code again, with those lines now visible and marked with arrows:

gems/activerecord-2.3.2/lib/active_record/validations.rb

module ActiveRecord
  module Validations

    def self.included(base)
      base.extend ClassMethods
      base.class_eval do
        alias_method_chain :save, :validation
        alias_method_chain :save!, :validation
      end

      # ...

    end

When ActiveRecord::Base includes the Validations module, the marked lines reopen Base and call a method named alias_method_chain. Let me show you a quick example to explain what alias_method_chain does.

The Reason ...