Do refinements apply only to instance methods?

Question

I'm trying to understand Ruby's refinements feature, and I've encountered a scenario I don't understand.

Take this example code:

class Traveller
  def what_are_you
    puts "I'm a Backpacker"
  end

  def self.preferred_accommodation
    puts "Hostels"
  end
end


module Refinements
  module Money
    def what_are_you
      puts "I'm a cashed-up hedonist!"
    end

    module ClassMethods
      def preferred_accommodation
        puts "Expensive Hotels"
      end
    end

    def self.included(base)
      base.extend ClassMethods
    end
  end

  refine Traveller do
    include Money
  end
end

Now, when I do this in the REPL:

Traveller.new.what_are_you         # => I'm a Backpacker
Traveller.preferred_accommodation  # => Hostels

using Refinements

Traveller.new.what_are_you         # => I'm a cashed-up hedonist!
Traveller.preferred_accommodation  # => Hostels (???)

Why is #what_are_you refined, but .preferred_accommodation is not?

Answer 1

As @MasashiMiyazaki explained, you have to refine two classes: Traveller and Traveller's singleton class. That actually allows you to simplify your code quite a bit:

module Money
  refine Traveller do
    def what_are_you
      puts "I'm a cashed-up hedonist!"
    end
  end

  refine Traveller.singleton_class do
    def preferred_accommodation
      puts "Expensive Hotels"
    end
  end
end

Traveller.new.what_are_you         #=> I'm a Backpacker
Traveller.preferred_accommodation  #=> Hostels

using Money
Traveller.new.what_are_you         #=> I'm a cashed-up hedonist!
Traveller.preferred_accommodation  #=> Expensive Hotels

Moreover, by putting the above three statements in a module, the refined versions of the two methods are confined to that module:

module M
  using Money
  Traveller.new.what_are_you         #=> I'm a cashed-up hedonist!
  Traveller.preferred_accommodation  #=> Expensive Hotels
end

Traveller.new.what_are_you           #=> I'm a Backpacker
Traveller.preferred_accommodation    #=> Hostels

Answer 2

You need to call refine Traveller with singleton_class scope to overwrite class methods. By adding the following code to your Refinements module instead of self.included, you can get the expected result.

module Refinements
  refine Traveller.singleton_class do
    include Money::ClassMethods
  end
end

This article(http://timelessrepo.com/refinements-in-ruby) will help you to understand Refinements more.