Callbacks overview in Ruby on Rails - Mini Project: Build & Apply

Callbacks overview in Rails

📖 Scenario: You are building a simple Rails model to manage books in a library. You want to automatically set some values and run checks when books are created or updated.

🎯 Goal: Learn how to use Rails callbacks to run code before and after saving a record.

📋 What You'll Learn

Create a Rails model with attributes

Add a callback to set a default value before saving

Add a callback to print a message after saving

Use the correct Rails callback methods

💡 Why This Matters

🌍 Real World

Callbacks in Rails models help automate common tasks like setting default values or logging changes without repeating code.

💼 Career

Understanding callbacks is essential for Rails developers to write clean, maintainable code that reacts to data changes automatically.

1

Create the Book model with attributes

Create a Rails model class called Book that inherits from ApplicationRecord and has attributes title and status.

Ruby on Rails

class Book < ApplicationRecord
  # Your code here
end

Hint

Use attr_accessor to define attributes for this example.

2

Add a before_save callback to set default status

Add a before_save callback in the Book model that sets status to 'available' if it is nil.

Ruby on Rails

class Book < ApplicationRecord
  attr_accessor :title, :status

  # Your code here
end

Hint

Use before_save :set_default_status and define the method set_default_status.

3

Add an after_save callback to print a message

Add an after_save callback in the Book model that calls a method print_saved_message which prints 'Book saved successfully'.

Ruby on Rails

class Book < ApplicationRecord
  attr_accessor :title, :status

  before_save :set_default_status
  # Your code here

  private

  def set_default_status
    self.status ||= 'available'
  end
end

Hint

Use after_save :print_saved_message and define the method print_saved_message with a puts statement.

4

Complete the Book model with callbacks

Ensure the Book model includes attr_accessor :title, :status, the before_save :set_default_status callback, and the after_save :print_saved_message callback with their respective methods.

Ruby on Rails

class Book < ApplicationRecord
  attr_accessor :title, :status

  before_save :set_default_status
  after_save :print_saved_message

  private

  def set_default_status
    self.status ||= 'available'
  end

  def print_saved_message
    puts 'Book saved successfully'
  end
end

# Model complete with callbacks

Hint

Review all previous steps and ensure the model has all parts combined correctly.

Practice

1. What is the main purpose of callbacks in Rails models?

easy

A. To run code automatically at specific points in a model's life

B. To manually call methods from controllers

C. To style the HTML output of views

D. To handle database migrations

Solution

1. Step 1: Understand what callbacks do

   Callbacks are hooks that run code automatically during certain model events like saving or deleting.

2. Step 2: Identify the correct purpose

   Callbacks help automate tasks at specific points in a model's lifecycle, not manual calls or styling.

3. Final Answer:

   To run code automatically at specific points in a model's life -> Option A

4. Quick Check:

   Callbacks = automatic code run [OK]

Hint: Callbacks run code automatically at model events [OK]

Common Mistakes:

• Confusing callbacks with controller methods
• Thinking callbacks style views
• Mixing callbacks with database migrations

2. Which of the following is the correct syntax to define a callback that runs before saving a record?

easy

A. on_save :method_name

B. after_save :method_name

C. before_save do method_name end

D. before_save :method_name

Solution

1. Step 1: Recall callback syntax

   Rails uses before_save :method_name to run a method before saving a record.

2. Step 2: Check other options

   after_save runs after saving, on_save is invalid, and the block syntax is incorrect here.

3. Final Answer:

   before_save :method_name -> Option D

4. Quick Check:

   before_save syntax = before_save :method_name [OK]

Hint: Use before_save :method_name to run code before saving [OK]

Common Mistakes:

• Using after_save instead of before_save
• Trying to use on_save which doesn't exist
• Incorrect block syntax for callbacks

3. Given this model code:

class User < ApplicationRecord
  before_create :set_default_role

  def set_default_role
    self.role ||= 'guest'
  end
end

What will be the role of a new User if none is set before saving?

medium

A. 'guest'

B. 'admin'

C. nil

D. Raises an error

Solution

1. Step 1: Understand before_create callback

   The before_create callback runs the method set_default_role before the record is created in the database.

2. Step 2: Analyze set_default_role method

   The method sets self.role to 'guest' only if it is currently nil or falsey (using ||=).

3. Final Answer:

   'guest' -> Option A

4. Quick Check:

   before_create sets role to 'guest' if none set [OK]

Hint: before_create sets default values if none exist [OK]

Common Mistakes:

• Assuming role stays nil if not set
• Confusing before_create with after_create
• Thinking it raises an error without role

4. Identify the error in this callback definition:

class Post < ApplicationRecord
  before_save :check_title

  def check_title
    if title.blank?
      errors.add(:title, "can't be blank")
      throw :abort
    end
  end
end

medium

A. Using throw :abort is incorrect to stop save

B. No error, this is correct usage

C. Callback method should be private

D. errors.add should be errors.add_error

Solution

1. Step 1: Review callback usage

   The before_save callback runs check_title before saving.

2. Step 2: Check error handling

   Adding errors and using throw :abort is the correct way to stop saving if validation fails.

3. Final Answer:

   No error, this is correct usage -> Option B

4. Quick Check:

   throw :abort stops save correctly [OK]

Hint: Use throw :abort to halt save in callbacks [OK]

Common Mistakes:

• Thinking throw :abort is invalid
• Believing errors.add needs a different method
• Assuming callback methods must be private

5. You want to run a method log_update only after a record is updated but not created. Which callback should you use?

hard

A. before_save

B. after_save

C. after_update

D. before_update

Solution

1. Step 1: Understand callback timing

   after_save runs after both create and update, but after_update runs only after updates.

2. Step 2: Choose callback for updates only

   Since you want to run log_update only after updates, after_update is the correct choice.

3. Final Answer:

   after_update -> Option C

4. Quick Check:

   after_update runs only after updates [OK]

Hint: Use after_update for actions only after updates [OK]

Common Mistakes:

• Using after_save which runs on create too
• Choosing before_update which runs before update
• Confusing before_save with after_update