Association callbacks in Ruby on Rails - Mini Project: Build & Apply

Association Callbacks in Rails

📖 Scenario: You are building a simple blog application where each Post can have many Comments. You want to automatically update a comments_count attribute on the Post model whenever a comment is created or destroyed.

🎯 Goal: Build a Rails model setup that uses association callbacks to update the comments_count on the Post model whenever comments are added or removed.

📋 What You'll Learn

Create a Post model with a comments_count integer attribute initialized to 0

Create a Comment model that belongs to Post

Use after_add and after_remove association callbacks on Post to update comments_count

Ensure the comments_count is incremented when a comment is added and decremented when a comment is removed

💡 Why This Matters

🌍 Real World

Association callbacks are useful in apps where related data needs automatic updates, like counting comments, likes, or tags.

💼 Career

Understanding association callbacks helps you maintain data integrity and write cleaner Rails models, a key skill for Rails developers.

1

Create the Post and Comment models with association

Create a Post model with a comments_count attribute set to 0 and a has_many :comments association. Also create a Comment model with a belongs_to :post association.

Ruby on Rails

# Create Post and Comment models with associations

class Post < ApplicationRecord
  # Your code here
end

class Comment < ApplicationRecord
  # Your code here
end

Need a hint?

Use has_many :comments in Post and belongs_to :post in Comment. Use attribute :comments_count, :integer, default: 0 to set default.

2

Add association callback configuration on Post

In the Post model, add after_add and after_remove callbacks on the comments association. Use method names increment_comments_count and decrement_comments_count respectively.

Ruby on Rails

class Post < ApplicationRecord
  has_many :comments, after_add: :increment_comments_count, after_remove: :decrement_comments_count
  attribute :comments_count, :integer, default: 0

  # Your code here
end

class Comment < ApplicationRecord
  belongs_to :post
end

Need a hint?

Add after_add: :increment_comments_count and after_remove: :decrement_comments_count to has_many :comments. Define empty methods increment_comments_count and decrement_comments_count.

3

Implement the increment and decrement methods

In the Post model, implement the increment_comments_count(comment) method to increase comments_count by 1 and save the record. Implement decrement_comments_count(comment) to decrease comments_count by 1 and save the record.

Ruby on Rails

class Post < ApplicationRecord
  has_many :comments, after_add: :increment_comments_count, after_remove: :decrement_comments_count
  attribute :comments_count, :integer, default: 0

  # Your code here
end

class Comment < ApplicationRecord
  belongs_to :post
end

Need a hint?

Use self.comments_count += 1 and save inside increment_comments_count. Use self.comments_count -= 1 and save inside decrement_comments_count.

4

Complete the Post model with association callbacks

Ensure the Post model has the full association with after_add: :increment_comments_count and after_remove: :decrement_comments_count callbacks, and the methods increment_comments_count and decrement_comments_count correctly update and save comments_count.

Ruby on Rails

class Post < ApplicationRecord
  has_many :comments, after_add: :increment_comments_count, after_remove: :decrement_comments_count
  attribute :comments_count, :integer, default: 0

  # Confirm all methods and callbacks are present
end

class Comment < ApplicationRecord
  belongs_to :post
end

Need a hint?

Review the Post model to confirm the association callbacks and methods are correctly implemented.