Metaprogramming Ruby 2

ActiveSupport::Concern

ActiveSupport::Concern encapsulates the include-and-extend trick and fixes the problem of chained inclusions. A module can get this functionality by extending Concern and defining its own ClassMethods module:

part2/using_concern.rb

require 'active_support'

module MyConcern
  extend ActiveSupport::Concern

  def an_instance_method; "an instance method"; end

  module ClassMethods
    def a_class_method; "a class method"; end
  end
end

class MyClass
  include MyConcern
end

MyClass.new.an_instance_method  # => "an instance method"
MyClass.a_class_method          # => "a class method"

In the rest of this chapter I’ll use the word “concern” with a lowercase C to mean “a module that extend ...