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 Interakt.co").deliver_now
    end

  end

Mailer class:-

  class MyAwesomeMailer < ActionMailer::Base

    def send_welcome_email(_user, _message)
      @user    = _user
      @message = _message
    end

  end

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 Interakt.co").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.
```
  MyAwesomeMailer.new.send_welcome_email(self, "Welcome to Interakt.co").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.
e.g:-
```
    class MyAwesomeClass

      private

      # Instance method
      def my_instance_method
        puts "I am not public any more"
      end

      # Class method
      def self.my_class_method
        puts "I am still public method"
      end

    end

    MyAwesomeClass.my_class_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 Interakt.co").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)
          MessageDelivery.new(self, method_name, *args)
        else
          super
        end
      end

    end

  end

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
    end

    def __getobj__ #:nodoc:
      @obj ||= @mailer.send(:new, @mail_method, *@args).message
    end

    # Returns the Mail::Message object
    def message
      __getobj__
    end

    def deliver_now
      message.deliver
    end
  end

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
  end

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

Share in your circles :)

Tricky Rails Mailer

How Rails ActionMailer trick methods

A bulb flashed ;) :-

Getting my Hands Dirty:-

Make class method private:-

Getting the idea:-

Flow(How it works internally):-

Happy Coding :)