Ruby on Rails: alias_method_chain, what exactly does it do?

Question

I\'ve tried reading through various blog posts that attempt to explain alias_method_chain and the reasons to use it and not use it. In particular, I took heed to:

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Answer 1

I'm not sure if it's gone out of style with Rails 3 or not, but it is still actively used in versions before that.

You use it to inject some functionality before (or after) a method is called, without modifying any place that calls that method. See this example:

module SwitchableSmtp
  module InstanceMethods
    def deliver_with_switchable_smtp!(mail = @mail)
      unless logger.nil?
        logger.info  "Switching SMTP server to: #{custom_smtp.inspect}" 
      end
      ActionMailer::Base.smtp_settings = custom_smtp unless custom_smtp.nil?
      deliver_without_switchable_smtp!(mail = @mail)
    end
  end
  def self.included(receiver)
    receiver.send :include, InstanceMethods
    receiver.class_eval do
      alias_method_chain :deliver!, :switchable_smtp
    end
  end
end

That's an addition to ActionMailer to allow swapping out of the SMTP settings on each call to deliver!. By calling alias_method_chain you are able to define a method deliver_with_switchable_smtp! in which you do your custom stuff, and call deliver_without_switchable_smtp! from there when you're done.

alias_method_chain aliases the old deliver! to your new custom method, so the rest of your app doesn't even know deliver! now does your custom stuff too.