Extending your include knowledge of Ruby

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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!

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