Many-to-many with has_many through in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding the association output

Given the following Rails models, what will user.posts return after creating the associations?

class User < ApplicationRecord
  has_many :user_posts
  has_many :posts, through: :user_posts
end

class Post < ApplicationRecord
  has_many :user_posts
  has_many :users, through: :user_posts
end

class UserPost < ApplicationRecord
  belongs_to :user
  belongs_to :post
end

# Setup
user = User.create(name: 'Alice')
post1 = Post.create(title: 'Hello')
post2 = Post.create(title: 'World')
UserPost.create(user: user, post: post1)
UserPost.create(user: user, post: post2)

A An array containing only post1 object

B An empty array

C An array containing only post2 object

D An array containing post1 and post2 objects

💡 Hint

Think about how has_many through works to connect user and posts via user_posts.

The user has two associated posts through the user_posts join table, so user.posts returns both post1 and post2 objects.

📝 Syntax

intermediate

Correct syntax for has_many through association

Which of the following is the correct way to declare a many-to-many association between Author and Book using a join model Authorship in Rails?

A

class Author < ApplicationRecord
  has_many :books
  has_many :authorships, through: :books
end

B

class Author < ApplicationRecord
  has_many :authorships
  has_many :books, through: :authorships
end

C

class Author < ApplicationRecord
  has_many :authorships, through: :books
  has_many :books
end

D

class Author < ApplicationRecord
  has_many :books, through: :authorships
end

💡 Hint

Remember the join model should be declared first, then the through association.

Option B correctly declares the join model association first, then the has_many through association. Other options have incorrect order or missing associations.

🔧 Debug

advanced

Why does user.posts return empty?

Given these models:

class User < ApplicationRecord
  has_many :user_posts
  has_many :posts, through: :user_posts
end

class Post < ApplicationRecord
  has_many :user_posts
  has_many :users, through: :user_posts
end

class UserPost < ApplicationRecord
  belongs_to :user
  belongs_to :post
end

# Data
user = User.create(name: 'Bob')
post = Post.create(title: 'Rails Tips')
# Missing association creation here

puts user.posts.inspect

Why does user.posts return an empty array?

A Because no UserPost record links the user and post

B Because the has_many :posts association is missing

C Because the User model should use has_and_belongs_to_many instead

D Because the Post model lacks a belongs_to :user

💡 Hint

Check if the join table has any records connecting user and post.

The join table UserPost must have a record linking the user and post. Without it, user.posts returns empty.

❓ state_output

advanced

Effect of destroying a join record

If a UserPost record linking a user and a post is destroyed, what happens to user.posts?

A user.posts still includes the post because the association caches it

B The post record is deleted from the database

C The post is removed from user.posts but the post record remains in the database

D An error is raised when accessing user.posts

💡 Hint

Think about what destroying a join record affects in a many-to-many association.

Destroying the join record removes the link, so user.posts no longer includes that post, but the post itself still exists.

🧠 Conceptual

expert

Why use has_many through instead of has_and_belongs_to_many?

Which is the main advantage of using has_many :through over has_and_belongs_to_many in Rails many-to-many associations?

A has_many through allows adding extra attributes to the join model

B has_and_belongs_to_many supports validations on the join table

C has_many through automatically creates the join table without migration

D has_and_belongs_to_many is deprecated and no longer works

💡 Hint

Think about what you can do with the join model in has_many through.

has_many through uses a join model where you can add extra fields and validations, unlike has_and_belongs_to_many which uses a simple join table.