Rails 4 ActiveSupport::Concern is a very cool feature to organize your module in such a way that when you include module in a class you don’t need to write extra callbacks to extend or include methods in your class. This feature is a great way to keep your code organized and DRY. This handles module dependencies also.

You just need to require activesupport/concern and need to extend your module with ActiveSupport::Concern. When you include your module in a class, included block will be executed with in class context, ClassMethods block will be added to class itself and methods inside will become class methods, other methods outside included and ClassMethods block will be included as instance methods. eg.

module Commentable
  extend ActiveSupport::Concern

  included do         # This block will be executed within class itself
    has_many :comments, as: :commentable, dependent: :destroy
    scope :with_comments, ->{ joins(:comments) }
  end 

  module ClassMethods # Methods within this black will be added to class methods
    def comment_search(params)
      .....
    end
  end

  def comment_titles  # This method will be added to instance methods
    comments.map(&:title) 
  end      
  def comment_users   # This method will be added to instance methods
    comments.map(&:username)
  end

end

This should be your module structure and when you include your module in class it will work. No method will break. eg.

Class Post
  include Commentable
end

Post.with_comments  
Post.comment_search({}) 

p = Post.find(1)
p.comments
p.comment_titles
p.comment_users