Include vs. Extend

Include and Extend allow us to take methods from a module and add them to an object. They work slightly differently from each other though. Let’s take a look…

Extend adds methods directly to an object

Extend adds methods to an object. It extends that object by adding new features to it.

class Hamster
end

module PetSkills
  def snuggle;end
end

Hamster.extend PetSkills

If you extend a class, you create a class method.

h = Hamster.new

Hamster.methods.include? :snuggle
# => true

h.methods.include? :snuggle
# => false

If you extend an instance of a class, you create an instance method, but only on that instance. You can extend any object like this.

h.extend PetSkills

h.methods.include? :snuggle
# => true

i = Hamster.new;
i.methods.include? :snuggle
# => false

You can call extend on any object to add methods to that object alone.

Include adds instance methods to a class

Include takes a more traditional approach. If you include a module in a class, the methods in the module will be added as instance methods, and will be available to all instances of that class.

class Gerbil
  include PetSkills
end

g = Gerbil.new

Gerbil.methods.include? :snuggle
# => false

g.methods.include? :snuggle
# => true

Upshot

Extend will add methods to an object, and only to that object. If we extend a class we get class methods.

Include will include methods from a module into a class, those methods become instance methods for objects of that type.