Callbacks overview in Ruby on Rails

Introduction

Callbacks let you run code automatically at certain points in an object's life. They help keep your code organized and avoid repeating yourself.

You want to set a default value before saving a record.

You need to clean or format data before saving it.

You want to send a notification right after a record is created.

You want to check or prevent something before deleting a record.

You want to log changes automatically when a record updates.

Syntax

Ruby on Rails

class ModelName < ApplicationRecord
  before_save :method_name
  after_create :another_method

  private

  def method_name
    # code to run before saving
  end

  def another_method
    # code to run after creating
  end
end

Callbacks are methods that run automatically at specific moments like before saving or after creating a record.

Use before_ or after_ prefixes to specify when the callback runs.

Examples

This runs the normalize_name method before saving the record.

Ruby on Rails

before_save :normalize_name

This runs the send_welcome_email method right after a new record is created.

Ruby on Rails

after_create :send_welcome_email

This runs check_dependencies before deleting a record to make sure it is safe to delete.

Ruby on Rails

before_destroy :check_dependencies

Sample Program

This example shows a User model with two callbacks. Before saving, it capitalizes the user's name. After creating, it prints a welcome message.

Ruby on Rails

class User < ApplicationRecord
  before_save :capitalize_name
  after_create :welcome_message

  private

  def capitalize_name
    self.name = name.capitalize
  end

  def welcome_message
    puts "Welcome, #{name}! Your account was created."
  end
end

# Simulate creating a user
user = User.new(name: 'alice')
user.save

Important Notes

Callbacks help keep your code clean by running code automatically at key moments.

Be careful not to put too much logic in callbacks to keep your code easy to understand.

Summary

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

Use before_ and after_ prefixes to control when they run.

They help keep your code organized and DRY (Don't Repeat Yourself).

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