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.

Practice

1. What is the main purpose of using has_many :through in Rails models?

easy

A. To create a many-to-many relationship using a join model

B. To create a one-to-one relationship between two models

C. To validate data before saving to the database

D. To automatically generate database migrations

Solution

1. Step 1: Understand relationship types in Rails

   Rails uses associations to connect models. has_many :through is specifically for many-to-many relationships.

2. Step 2: Identify the role of join models

   It uses a join model to connect two other models, allowing extra data on the connection.

3. Final Answer:

   To create a many-to-many relationship using a join model -> Option A

4. Quick Check:

   has_many :through = many-to-many with join model [OK]

Hint: Think: many-to-many needs a join model [OK]

Common Mistakes:

• Confusing has_many :through with has_one
• Thinking it automatically creates migrations
• Using it for one-to-one relationships

2. Which of the following is the correct way to declare a many-to-many association using has_many :through in the Author model?

easy

A. has_one :books, through: :authorships

B. has_many :books, through: :authorships

C. belongs_to :books, through: :authorships

D. has_many :authorships, through: :books

Solution

1. Step 1: Identify the correct association syntax

   The model that wants to access the other through a join model uses has_many :other_model, through: :join_model.

2. Step 2: Apply to Author model

   Author has many books through authorships, so has_many :books, through: :authorships is correct.

3. Final Answer:

   has_many :books, through: :authorships -> Option B

4. Quick Check:

   has_many :books, through: :authorships = correct syntax [OK]

Hint: Remember: has_many :target, through: :join_model [OK]

Common Mistakes:

• Reversing the join and target models
• Using belongs_to instead of has_many
• Using has_one for many-to-many

3. Given these models:

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

class Authorship < ApplicationRecord
  belongs_to :author
  belongs_to :book
end

class Book < ApplicationRecord
  has_many :authorships
  has_many :authors, through: :authorships
end

What will Author.first.books.count return if the first author has 3 books linked through authorships?

medium

A. 1

B. 0

C. 3

D. An error because of missing join table

Solution

1. Step 1: Understand the association setup

   Author has many books through authorships, so calling Author.first.books fetches all books linked via authorships.

2. Step 2: Count the linked books

   If the first author has 3 books linked, Author.first.books.count returns 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Author.first.books.count = number of linked books [OK]

Hint: Count returns linked records through join model [OK]

Common Mistakes:

• Assuming count returns authorships count
• Expecting zero without checking data
• Confusing belongs_to with has_many effects

4. You have these models:

class Student < ApplicationRecord
  has_many :enrollments
end

class Enrollment < ApplicationRecord
  belongs_to :student
  belongs_to :course
end

class Course < ApplicationRecord
  has_many :enrollments
  has_many :students, through: :enrollments
end

But when you run Student.first.courses, you get an error: undefined method 'courses'. What is the likely cause?

medium

A. The has_many :courses, through: :enrollments line is missing in Student model

B. The Enrollment model is missing belongs_to :course

C. The Course model should use belongs_to :student instead of has_many

D. The database table for enrollments does not exist

Solution

1. Step 1: Check Student model associations

   The error says courses method is undefined, meaning Student model lacks has_many :courses, through: :enrollments.

2. Step 2: Confirm other associations

   Enrollment and Course models are correct; missing association in Student causes the error.

3. Final Answer:

   The has_many :courses, through: :enrollments line is missing in Student model -> Option A

4. Quick Check:

   Missing has_many through causes undefined method [OK]

Hint: Check if has_many :through is declared in the model [OK]

Common Mistakes:

• Assuming belongs_to missing causes undefined method
• Thinking Course model needs belongs_to Student
• Ignoring missing database tables as cause

5. You want to track the date when a User joins a Group using a many-to-many relationship with has_many :through. Which setup correctly allows storing this extra data?

hard

A. Add joined_on column directly to users table and use has_many :groups

B. Store joined_on in a separate table unrelated to the join

C. Use has_and_belongs_to_many :groups without a join model

D. Create a join model Membership with user_id, group_id, and joined_on columns, then use has_many :groups, through: :memberships in User

Solution

1. Step 1: Understand storing extra data in many-to-many

   Extra data on the connection requires a join model with its own columns.

2. Step 2: Apply to User and Group

   Create a join model Membership with user_id, group_id, and joined_on columns. Then User uses has_many :groups, through: :memberships.

3. Final Answer:

   Create a join model Membership with user_id, group_id, and joined_on columns, then use has_many :groups, through: :memberships in User -> Option D

4. Quick Check:

   Extra data needs join model with columns [OK]

Hint: Use join model to store extra data in many-to-many [OK]

Common Mistakes:

• Adding extra columns to main models instead of join model
• Using has_and_belongs_to_many which lacks extra data support
• Storing extra data in unrelated tables