Included hook in Ruby

Introduction

The included hook runs code automatically when a module is added to a class. It helps set up things right away without extra steps.

When you want to add methods or variables to a class as soon as a module is included.

When you need to run setup code automatically for classes using your module.

When you want to add class-level methods or extend behavior on inclusion.

When you want to keep your code organized by separating setup logic inside the module.

Syntax

Ruby

module MyModule
  def self.included(base)
    # code to run when included
  end
end

The included method is a class method of the module.

The base parameter is the class that includes the module.

Examples

This prints a message when the module is included in the Person class.

Ruby

module Greetings
  def self.included(base)
    puts "Greetings module included in #{base}!"
  end
end

class Person
  include Greetings
end

This adds a class method track to Device when Tracker is included.

Ruby

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

  module ClassMethods
    def track
      puts "Tracking started"
    end
  end
end

class Device
  include Tracker
end

Device.track

Sample Program

This program shows how the included hook prints a message and adds a class method log to Application.

Ruby

module Logger
  def self.included(base)
    puts "Logger included in #{base}"
    base.extend(ClassMethods)
  end

  module ClassMethods
    def log(message)
      puts "LOG: #{message}"
    end
  end
end

class Application
  include Logger
end

Application.log("App started")

Important Notes

The included hook runs only once per class when the module is included.

You can use base.extend inside included to add class methods.

This hook helps keep your modules flexible and easy to reuse.

Summary

The included hook runs code automatically when a module is included in a class.

It receives the class as a parameter to customize behavior.

Use it to add methods or setup code without extra steps.

Practice

1.

What does the included hook do in a Ruby module?

easy

A. It runs only when the module is extended, not included.

B. It runs code automatically when the module is included in a class.

C. It prevents the module from being included in any class.

D. It defines a method that must be called manually.

Solution

1. Step 1: Understand the purpose of the included hook

   The included hook is a special method in Ruby modules that runs automatically when the module is included in a class.

2. Step 2: Compare options with the definition

   Only It runs code automatically when the module is included in a class. correctly describes this automatic behavior. Other options describe manual calls, prevention, or extension, which are incorrect.

3. Final Answer:

   It runs code automatically when the module is included in a class. -> Option B

4. Quick Check:

   Included hook = automatic run on include [OK]

Hint: Remember: included runs automatically on module inclusion [OK]

Common Mistakes:

• Thinking included runs on extend instead of include
• Believing included requires manual method calls
• Confusing included with preventing inclusion

2.

Which of the following is the correct syntax to define an included hook inside a Ruby module?

module Example
  # What goes here?
end

easy

A. def self.included(base)
puts "Included in #{base}"
end

B. def included(base)
puts "Included in #{base}"
end

C. def included
puts "Included"
end

D. def self.include(base)
puts "Included in #{base}"
end

Solution

1. Step 1: Recall how to define class methods in modules

   The included hook must be a module method, so it needs to be defined with self.included.

2. Step 2: Check the method signature

   The method receives the including class as a parameter, so it must accept one argument (base).

3. Final Answer:

   def self.included(base) puts "Included in #{base}" end -> Option A

4. Quick Check:

   included hook = def self.included(base) [OK]

Hint: Use def self.included(base) for included hook syntax [OK]

Common Mistakes:

• Defining included without self (instance method instead of module method)
• Missing the base parameter
• Using def self.include instead of included

3.

What will be the output of this Ruby code?

module Greetings
  def self.included(base)
    puts "Greetings included in #{base}"
  end
end

class Person
  include Greetings
end

medium

A. SyntaxError

B. No output

C. Greetings included in Person

D. Greetings included in Greetings

Solution

1. Step 1: Identify when included hook runs

   The included hook runs automatically when the module is included in a class.

2. Step 2: Check the output inside the included method

   The method prints "Greetings included in #{base}", where base is the class Person.

3. Final Answer:

   Greetings included in Person -> Option C

4. Quick Check:

   included hook prints class name on include [OK]

Hint: included hook prints message with class name on include [OK]

Common Mistakes:

• Expecting no output because included is a hook
• Confusing base with module name
• Thinking included runs on class definition, not include

4.

Find the error in this Ruby code using the included hook:

module Logger
  def included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def log(msg)
      puts "Log: #{msg}"
    end
  end
end

class App
  include Logger
end

App.log("Start")

medium

A. The include statement should be extend.

B. The ClassMethods module should be included, not extended.

C. The log method should be an instance method, not class method.

D. The included method should be defined as self.included, not instance method.

Solution

1. Step 1: Check how included hook is defined

   The included hook must be a module method, defined as self.included, not an instance method.

2. Step 2: Understand the effect of incorrect definition

   Defining included without self means it won't run automatically on include, so base.extend won't happen.

3. Final Answer:

   The included method should be defined as self.included, not instance method. -> Option D

4. Quick Check:

   included hook must be self.included [OK]

Hint: Define included as self.included to run on module include [OK]

Common Mistakes:

• Defining included without self
• Confusing extend and include in hook
• Trying to call class method on instance

5.

You want to add a class method greet to any class that includes your module Friendly. Which is the best way to do this using the included hook?

module Friendly
  # Fill in the included hook
end

class Dog
  include Friendly
end

puts Dog.greet

hard

A. Define self.included(base) and inside it call base.extend with a module containing greet.

B. Define an instance method greet inside Friendly module.

C. Define a class method greet directly inside Friendly module without included hook.

D. Call include Friendly inside Dog.greet method.

Solution

1. Step 1: Understand how to add class methods via included hook

   To add class methods, define self.included(base) and call base.extend with a module containing those methods.

2. Step 2: Why other options fail

   Instance methods won't be class methods; defining class methods directly in module won't add them to including class; calling include inside method is incorrect.

3. Final Answer:

   Define self.included(base) and inside it call base.extend with a module containing greet. -> Option A

4. Quick Check:

   Use included hook + base.extend for class methods [OK]

Hint: Use included hook with base.extend to add class methods [OK]

Common Mistakes:

• Defining greet as instance method instead of class method
• Not using included hook to extend class
• Trying to call include inside a method