mixin_recipe.rb

# Create a file my_modeule.rb
# Include it in your application load path (Standard: app/lib )

# FILE CONTENTS BEGIN HERE
module MyModule
  def self.included(base)
    base.extend ClassMethods
    class << base
      attr_accessor :class_variable
    end
    attr_accessor :instance_variable
  end

  module ClassMethods
    # Options passed here will be overridden by those passed in the instance method
    def i_am_a_class_method(options = {})
      # Class variables available here
      self.class_variable
    end
  end

  module InstanceMethods
    # By default these methods are not available.
    # They are only added when we call the 'i_am_a_class_method' method.
    def i_am_an_instance_method(options = {})
      # All instance variables from class will be available here
      "Whatever the output yo desire!"
    end

    private
    
    def a_private_method
      # Manipulate stuff here
    end
  end
end

# The most important line. Include it to ActiveRecord::Base, so that
# Its available to all models
ActiveRecord::Base.send :include, MyModule

# FILE CONTENTS END HERE

# How to use it:

class MyModel < ActiveRecord::Base

  i_am_a_class_method :param_one => :something, :param_two => "something_else"

  # This one line will add all the instance methods you wrote in your module.

end

# Now simply:

MyModel.new.i_am_an_instance_method
# Whatever the output you desire!