定义为实例方法的类方法

Question

This is a working code from the Michael Hartle book. This is the mailer code in app/mailers/user_mailer.rb to create an activation mail for a user account:

class UserMailer < ActionMailer::Base
  def account_activation(user)
    @user = user
    mail to: user.email, subject: "Account activation"
  end
end

The preview of the mail is generated using the ruby file in test/mailers/previews/user_mailer_preview.rb :

class UserMailerPreview < ActionMailer::Preview
  def account_activation
    user = User.first
    user.activation_token = User.new_token
    UserMailer.account_activation(user)
  end
end

The account_activation method is defined as an instance method in user_mailer.rb . But it is used in preview generator as a class method. Did I misunderstand the code or is there something else going on here?

To anyone who have the same doubt

That's how ActionMailer works. Emails are defined as instance method in a class that extends ActionMailer::Base , but you access them as class methods.

Answer 1

That's how ActionMailer works. Emails are defined as instance method in a class that extends ActionMailer::Base , but you access them as class methods.

class MyMailer < ActionMailer::Base
  def my_email
  end
end

MyMailer.my_email
# and not MyMailer.new.my_email

This is a shortcut that will instantiate an instance of the ActionMailer class, invoke the corresponding email method and return an email message. This is the code that handles the call .

Answer 2

The Magic is behind deliver/deliver_now/deliver_later (or any other deliver method):

def deliver_now
  processed_mailer.handle_exceptions do
    message.deliver
  end
end

Looks like processed_mailer is the key method that we were looking for:

def processed_mailer
  @processed_mailer ||= @mailer_class.new.tap do |mailer|
    mailer.process @action, *@args
  end
end

This method creates the instance of the mailer, calls process method with @action argument (which is the name of the instance method) and with @args , which are the arguments passed to the class method and in the end it returns the created instance of the mailer.

So thats the reason that mailer methods are being declared like instance ones but get called like class ones.

Happy Learning :)