Using Module Mixins to Extend Classes and Objects in Ruby

The module and class below demonstrate how to use instance methods and class methods from “module mixins”. The thing to remember is scope. For instance, to use class methods within instance methods you need to call them from the class itself. This is accomplished something like this “self.class.class_method_you_want_to_call”. The example below should make it more clear. It would be natural to assume using “self.method_name” within your module mixin, because after all, instance methods just work when included this way. However, you need to take an extra step when you want to “extend” the behavior of a class.

To extend a class with a mixin you use the method “self.included(base)”. This method takes the parent class “base” as an argument. In the method body you use base.extend to attach class methods. Wrapping up class methods in a module is the easiest way to do it.

module Greeting
  # Instance Methods
  # Usage Martian.new.hello_world
  def hello_world
    "Martian says: Hello " + self.class.planet+"!"
  end

  # Class Methods
  # Usage: Martian.planet
  def self.included(base)
    base.extend ClassMethods
  end
  module ClassMethods
    def planet
      "world"
    end
  end
end

class Martian
  include Greeting
  def self.location
    "Martian lives on "+self.planet
  end
end

p Martian.new.hello_world
p Martian.location

Make Rails Lib Module Methods Available to Views

If you create a module in the lib/ directory of your Rails application you won’t have access to those methods in your views. If you don’t want to put those methods in a helper file, you need to add a method to your module that makes them available for you views like so…

#in lib/my_module_name.rb
module MyModuleName
  def my_method_for_views
     #logic...
  end
  def self.included(base)
    base.send :helper_method, :my_method_for_views if base.respond_to? :helper_method
  end
end