Reason for calling an instance method on a class in Ruby?

Question

I would like to know is there any specific reason for doing this or is this a silly mistake done by someone (or is it something else that I am not understanding).

class SomeMailer < ActionMailer::Base
  def first_method(user)
    mail(to: user.email, subject: "Testing")
  end
end

This method is called at some other place in the code as follows

SomeMailer.first_method(user).deliver

Answer 1

ActionMailer::Base classes are weird... Yes, you do indeed call instance methods on the class - which obviously won't work for 'normal' classes!

But there's some meta-programming magic under the hood :

module ActionMailer
  class Base < AbstractController::Base
    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

If you look through the rails documentation , you'll see that calling instance methods on the class is, strangely, the normal thing to do for mailers.

Answer 2

This is how rails is intended to work. It is also mention in rails guides that

You never instantiate your mailer class. Rather, you just call the method you defined on the class itself.

Rails do the internal processing by invoking method_missin . Basically, any action method defined in mailer class will be intercepted by method_missing and will return an instance of MessageDelivery, otherwise it runs the default implementation. And where do action methods come from? ActionMailer::Base inherits from AbstractController::Base, so it works exactly the same as for controllers - it returns a set of public instance methods of a given class.

Rails itself encourage this behavior. For more information, you can refer this link

Answer 3

I will try to answer this since I have come across similar situations myself while working on existing code.

The instance methods like this in a class help when you do call backs on a class. For example, if you want to perform some action on a object that was created from that class.

Say you have another class User and you want to send an email to a user immediately after creating a new user. In that case you can call this method on that object by doing

after_save :method_name