In Ruby, when you include a module in a class, the methods in the module become instance methods on that class. If you extend a class with a module, the module's methods become class methods. We can use the included hook that Ruby provides to automatically extend any class that includes our module.

With that in mind, let’s extend ActiveRecord with the ActsAsLucky module and introduce its instance methods and class methods accordingly:

module ActsAsLucky
  def acts_as_lucky
    include InstanceMethods
    extend ClassMethods
  end
  
  module InstanceMethods
    # The lucky number should be the last digit of the model's ID
    # Example:
    #   user.id           #=> 123
    #   user.lucky_number #=> 3
    def lucky_number
      id.to_i % 10
    end
  end

  module ClassMethods
    def find_by_lucky_number(number)
      where("id % 10 = ?", number)
    end
  end
end

ActiveRecord::Base.extend ActsAsLucky

then create a file config/initializers/acts_as_lucky.rb with the following:

require Rails.root.join('lib/acts_as_lucky')

which lets us add the following to an assumed User model

class Person < ActiveRecord::Base
  acts_as_lucky
end

and now run code like

>> 27.times { User.create }
=> 27

>> user = User.last
=> #<User id: 27>

>> user.id
=> 27

>> user.lucky_number
=> 7

>> User.find_by_lucky_number(7)
=> [#<User id: 7>, #<User id: 17>, #<User id: 27>]

When you’re ready to move your library out to a proper Ruby gem of its own for easier distribution/fame and fortune, then change directories to wherever you write new projects and create the gem:

bundle gem acts_as_lucky

Take a look at the results and drop in your code from earlier. Refer to rubygems.org for how best to structure your gem and publish it. Once it’s published, you can remove lib/acts_as_lucky.rb and its initializer and add your gem to your Gemfile like all the rest.

Rails generators are also game for creation; see Rails Guides for a walkthrough.