Example for adding functionality to instances of a class which needs help from class methods of the same class. Use two modules - one for instance methods and one for class methods.

automated_class_instance_collaboration.rb

#!/usr/bin/ruby -w

module Foo
  module Foo4Class
    def reset_instance_count
      @instance_count = 0
    end

    def new(*a, &b)
      super.tap { @instance_count += 1 }
    end

    def instance_count
      @instance_count
    end
  end

  def shout_instance_count
    printf "There have been %d of us created!\n", self.class.instance_count
  end

  # setup mechanics
  def self.included(cl)
    cl.extend(Foo4Class).reset_instance_count
  end
end


class Bar
  include Foo
end

Array.new(10) { Bar.new }.each(&:shout_instance_count)

https://chriskottom.com/blog/2015/03/testing-ruby-mixins-in-isolation/
http://stackoverflow.com/questions/1542945/testing-modules-in-rspec
http://api.rubyonrails.org/classes/ActiveSupport/Concern.html