Understanding include vs extend in Ruby on Rails

When working with Ruby on Rails, understanding how to share functionality between classes is essential. Two key tools for achieving this are include and extend, which allow you to incorporate methods from modules into your classes. Although they may seem similar, they serve distinct purposes.

What is include?

The include keyword is used to add methods from a module as instance methods to a class. This means objects created from the class can access these methods.

Example of include:

module Greetable
  def greet
    "Hello from an instance!"
  end
end

class User
  include Greetable
end

user = User.new
puts user.greet  # Output: "Hello from an instance!"

In this example, greet becomes an instance method accessible from the User object.

What is extend?

The extend keyword adds methods from a module as class methods to a class. As a result, the class itself can call these methods.

Example of extend:

module Greetable
  def greet
    "Hello from the class!"
  end
end

class User
  extend Greetable
end

puts User.greet  # Output: "Hello from the class!"

In this case, greet becomes a class method accessible directly from the User class.

Combining include and extend

Sometimes, you may want both class-level and instance-level methods from the same module. You can achieve this by using self.included to extend the class when it includes the module.

Example of include with extend:

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

  module ClassMethods
    def class_greet
      "Hello from class method!"
    end
  end

  def instance_greet
    "Hello from instance method!"
  end
end

class User
  include Greetable
end

user = User.new
puts user.instance_greet   # Output: "Hello from instance method!"
puts User.class_greet      # Output: "Hello from class method!"

This approach allows the module to define both class-level and instance-level behaviors.

Key Differences Between include and extend

Conclusion

Understanding the difference between include and extend is crucial for writing clean, modular, and reusable code in Ruby on Rails applications. Use include for instance methods, extend for class methods, and combine them when you need both. Mastering these patterns will make your Rails applications more organized and efficient.