Ruby class vs instance method confusion

Question

Given the class:

class UserMailer < ActionMailer::Base
  default from: "do-not-reply@mark.com"
  def contact_email(contact)
    @contact = contact
    mail(to: 'm@mark.com', from: @contact.email, subject: "Website Contact")
  end
end

and the following test code:

c = Contact.new
UserMailer.contact_email(c)

How is it that this code works? I thought my contact_email was an instance method, but it is being invoked as a class method, and it works.

thanks for your help - as I learn Ruby and Rails :)

-mark

score 5 · Accepted Answer · answered Jan 13 '15 at 19:14

You're entirely correct that this looks wrong at first glance.

It works because there is a method_missing on the class (see source) that looks like this

  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

action_methods is basically the names of the methods of your mailer that correspond to emails that can be sent, and MessageDelivery is a little proxy class that will eventually do

YourMailer.new.send(:contact_mailer, ...)

Off the top of my head I'm not entirely sure why this is done this way, but the basic class method to instance method proxying has been around in one form or another since the very early days of actionmailer

thank you, very helpful. I got a quick lesson in Ruby's method_missing and one in Rails' mailer with these answers!! — Mark Feinholz, Jan 13 '15 at 21:24

Ajay · Answer 2 · 2015-01-13T19:51:15.160

Check source

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

Sample implementation:

class MyMailer
  def self.method_missing(method, *args)
    puts "Here, I can call any instance method" 
  end

  def sending_mail_for_you 
    puts "I am actually sending mail for you"
  end  

end
#notice, fake_method is not defined in the MyMailer class.
MyMailer.fake_method
This will give output: 
=> "Here, I can call any instance method"
"I am actually sending mail for you"

ActionMailer::Base does something like above code. Even we don't have any such method called "fake_method" still when the method_missing section is executed, it internally gives call to your 'sending_mail_for_you' method.

Ruby class vs instance method confusion

2 Answers2