Tricky Rails Mailer

How Rails ActionMailer trick methods

  • ActionMailer in Rails, yooo its awesome it do tons of stuff and makes my life easy while it comes to sending email from Rails app.

  • If you don’t know much about it, you should check ActionMailer guide as well, explained pretty well.

  • A bulb flashed ;) :-

    • I was writing some code to send emails after user signs up:-

    Model class:-

      class User < ActiveRecord::Base
        after_commit :notify_user, on: :create
        def notify_user
          MyAwesomeMailer.send_welcome_email(self, "Welcome to").deliver_now

    Mailer class:-

      class MyAwesomeMailer < ActionMailer::Base
        def send_welcome_email(_user, _message)
          @user    = _user
          @message = _message
    • All of sudden a bulb flashed over my head and I realized send_welcome_email is an instance method and I called a class method here.
        MyAwesomeMailer.send_welcome_email(self, "Welcome to").deliver_now
    • I was confused and asked from couple of my senior but didn’t get exact answer, one tried to guess and said it might be using method_missing hook of Ruby.
  • Getting my Hands Dirty:-

    • Finally decided to get my hands dirty with source code for first time and trust me it was worth it :).

    • Once I cloned the Rails source code I jumped directly into ActionMailer::Base module and started browsing it.

    • First thing I tried with it was calling send_welcome_email like a instance method but it didn’t work, go and try it you will get the same exception., "Welcome to").deliver_now
        NoMethodError: private method `new' called for MyAwesomeMailer:Class
  • Make class method private:-

    • Rails mark new method of mailer as private method so you can’t call it using dot(.) operator.

    • Another thing that I analyzed was new is a class method and if I use private or protected it will not work for class methods.


        class MyAwesomeClass
          # Instance method
          def my_instance_method
            puts "I am not public any more"
          # Class method
          def self.my_class_method
            puts "I am still public method"
        ==> I am still public method
    • Callling MyAwesomeClass.my_class_method and it will work and its not a private method, it cleary indicates its not set as private method. Now the question is how to make class level methods private.

    • I came across a new method private_class_method for ruby, usage:-

      private_class_method :new

    It makes class methods private and Rails use it all the time.

  • Getting the idea:-

    • Coming back to our point when I call a class method how it calls the instance method with the same name.

    • Finally after messing my head around for some time I got the answer and that is its basically using method_missing hook of ruby and doing the stuff in backend.

  • Flow(How it works internally):-

    • When you call a class method e.g:-
      MyAwesomeMailer.send_welcome_email(self, "Welcome to").deliver_now
    • first thing is that(Class Method) method is not available there so method_missing will be called and this is the code inside method_missing hook method.
      class Base < AbstractController::Base
        class << self
          def method_missing(method_name, *args) # :nodoc:
            if action_methods.include?(method_name.to_s)
    , method_name, *args)
    • In code about that method is not called but created new object of MessageDelivery and yeah you guessed it right that class is doing the whole stuff, have a look on code:-
      class MessageDelivery < Delegator
        def initialize(mailer, mail_method, *args) #:nodoc:
          @mailer = mailer
          @mail_method = mail_method
          @args = args
        def __getobj__ #:nodoc:
          @obj ||= @mailer.send(:new, @mail_method, *@args).message
        # Returns the Mail::Message object
        def message
        def deliver_now
    • Here after initializing the object, when you call deliver_now to deliver your email and it calls message method wich call __getobj__ method.
      def __getobj__ #:nodoc:
        @obj ||= @mailer.send(:new, @mail_method, *@args).message
    • Here @mailer is containing you class i.e MyAwesomeMailer, @mail_method is containing method_name you called.
    • This code is basically creating a new instance of your MyAwesomeMailer class and running the method you called on it.

One conclusion of all that is Rails is doing a lot of magic behind the scene and helping developers to focus on business logic. Its very easy to begin with but it take some efforts to master in & out of it :).

Happy Coding :)

Written on May 22, 2015

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