Extending your include knowledge of Ruby

extend and include are two confusing methods, but they go hand in hand like Lychee and Vodka (humm…). Hopefully I’ll try to clarify this as short as possible.

include(a_module) is called inside a class, and adds module methods as instance methods.

extend(a_module) adds all module methods to the instance it is called on.

Here’s a quick example:

  module InstanceMethods
    def an_instance_method
      "You called an_instance_method on #{self.class}"
    end
  end
  module ClassMethods
    def a_class_method
      "You called a_class_method_from_module on #{self}"
    end
  end

  class MyClass
    include InstanceMethods
    extend ClassMethods
  end

  my_class = MyClass.new
  puts my_class.an_instance_method # >> You called an_instance_method on MyClass
  puts MyClass.a_class_method      # >> You called a_class_method on MyClass

  obj = Object.new
  obj.extend InstanceMethods
  puts obj.an_instance_method      # >> You called an_instance_method on Object

As you can see, because extend is called inside the class (MyClass), and because Class is an object in Ruby, it adds module methods as class methods.

A common scenario for doing all this it to split large classes in several modules (and files), like is ActiveRecord::Base. To get rid of the extend ClassMethods you can take advantage of the included callback and rewrite to:

  module InstanceMethods
    def self.included(base)
      base.extend(ClassMethods)
    end
  end

  class MyClass
    include InstanceMethods
  end

Got any tips ? Share with us the way you use include and extend!

Chris point out another great article about include and exclude that is even more easier to understand for C#/Java programmer, thanks Chris!