has_many relationship in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Rails code output when accessing associated records?

Given the following models, what will user.posts.count return after running the code below?

Ruby on Rails

class User < ApplicationRecord
  has_many :posts
end

class Post < ApplicationRecord
  belongs_to :user
end

user = User.create(name: "Alice")
user.posts.create(title: "First Post")
user.posts.create(title: "Second Post")

result = user.posts.count

A 2

B 0

C 1

D Raises an error

💡 Hint

Remember that has_many sets up a collection of associated records.

The user has two posts created and saved. Calling user.posts.count returns the number of associated posts, which is 2.

📝 Syntax

intermediate

Which option correctly defines a has_many association in Rails?

Select the correct syntax to define a has_many association from User to Post.

A

class User < ApplicationRecord
  has_many posts
end

B

class User < ApplicationRecord
  has_many 'posts'
end

C

class User < ApplicationRecord
  has_many :post
end

D

class User < ApplicationRecord
  has_many :posts
end

💡 Hint

The association name should be a symbol and plural for has_many.

Option D uses the correct syntax: has_many :posts. Option D misses the colon, C uses singular, and D uses quotes instead of a symbol.

🔧 Debug

advanced

Why does this has_many association not load related records?

Given these models, why does user.posts return an empty array even though posts exist in the database?

Ruby on Rails

class User < ApplicationRecord
  has_many :posts
end

class Post < ApplicationRecord
  belongs_to :author
end

user = User.first
posts = user.posts

A The posts table is empty, so no posts exist.

B The User model should use has_one instead of has_many.

C The Post model's belongs_to uses :author instead of :user, so the association is broken.

D The user variable is nil, so no posts can be loaded.

💡 Hint

Check if the foreign key and association names match between models.

The User model expects posts to belong to :user, but Post belongs_to :author, so Rails cannot link them. The association names must match for has_many and belongs_to.

❓ state_output

advanced

What is the value of post.user.name after this code runs?

Consider these models and code. What will post.user.name output?

Ruby on Rails

class User < ApplicationRecord
  has_many :posts
end

class Post < ApplicationRecord
  belongs_to :user
end

user = User.create(name: "Bob")
post = Post.create(title: "Hello", user: user)

result = post.user.name

A nil

B "Bob"

C Raises NoMethodError

D "Hello"

💡 Hint

The post belongs to the user created with name "Bob".

The post is linked to the user with name "Bob". Accessing post.user.name returns "Bob".

🧠 Conceptual

expert

Which statement about has_many associations is true?

Select the correct statement about Rails has_many associations.

A A has_many association always requires a foreign key in the associated model's table.

B has_many associations automatically create database indexes on foreign keys.

C has_many associations can only be used with models that have a belongs_to association back.

D has_many associations do not support dependent option for cascading deletes.

💡 Hint

Think about how Rails links models in a has_many relationship.

has_many requires the associated model's table to have a foreign key column pointing back to the owner model. Other options are false: indexes are not automatic, belongs_to is recommended but not strictly required, and dependent option is supported.

Practice

1. What does the has_many association in Rails represent?

easy

A. One model is linked to many instances of another model

B. One model belongs to exactly one other model

C. Models share a many-to-many relationship without a join table

D. It creates a database index automatically

Solution

1. Step 1: Understand the purpose of has_many

   The has_many association is used to link one model to many instances of another model.

2. Step 2: Compare with other associations

   belongs_to links one model to one other model, and many-to-many requires has_many :through or has_and_belongs_to_many.

3. Final Answer:

   One model is linked to many instances of another model -> Option A

4. Quick Check:

   has_many means one-to-many [OK]

Hint: Remember: has_many means one model owns many others [OK]

Common Mistakes:

• Confusing has_many with belongs_to
• Thinking has_many creates database indexes
• Mixing up many-to-many with has_many

2. Which of the following is the correct syntax to declare a has_many association in a Rails model?

easy

A. has_many => :comments

B. has_many = comments

C. has_many :comments

D. has_many('comments')

Solution

1. Step 1: Recall Rails association syntax

   Rails uses the syntax has_many :association_name without equals or arrows.

2. Step 2: Check each option

   has_many :comments uses the correct symbol and syntax. Options B, C, and D use invalid Ruby syntax for associations.

3. Final Answer:

   has_many :comments -> Option C

4. Quick Check:

   Correct Rails association syntax = has_many :comments [OK]

Hint: Use symbol and colon: has_many :model_name [OK]

Common Mistakes:

• Using equals sign instead of colon
• Using hash rockets => in association
• Calling has_many like a method with parentheses

3. Given these models:

class Author < ApplicationRecord
  has_many :books
end

class Book < ApplicationRecord
  belongs_to :author
end

What will author.books.count return if the author has 3 books?

medium

A. 3

B. 0

C. nil

D. An error because books is undefined

Solution

1. Step 1: Understand the association setup

   The Author model has many books, and each Book belongs to an author.

2. Step 2: Check the method call

   author.books.count counts all books linked to that author. If there are 3 books, it returns 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   author.books.count = number of linked books [OK]

Hint: Count returns how many related records exist [OK]

Common Mistakes:

• Expecting count to return nil or zero incorrectly
• Confusing belongs_to with has_many behavior
• Assuming books method is undefined

4. Identify the error in this code snippet:

class Library < ApplicationRecord
  has_many :books
end

class Book < ApplicationRecord
  has_many :library
end

medium

A. No error, code is correct

B. Library should use belongs_to :books

C. has_many should be replaced with has_one in Library

D. Book should use belongs_to :library, not has_many

Solution

1. Step 1: Check association pairing

   has_many on one side must pair with belongs_to on the other side for one-to-many.

2. Step 2: Identify the mistake

   The Book model incorrectly uses has_many :library. It should be belongs_to :library.

3. Final Answer:

   Book should use belongs_to :library, not has_many -> Option D

4. Quick Check:

   has_many pairs with belongs_to [OK]

Hint: has_many pairs with belongs_to, not has_many [OK]

Common Mistakes:

• Using has_many on both sides
• Swapping belongs_to and has_many
• Using singular instead of plural for has_many

5. You want to list all comments for a post and also count how many comments have the word 'help' in them. Given:

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

Which code correctly gets the count of comments containing 'help' for a post instance post?

hard

A. post.comments.find_all { |c| c.content.include?('help') }

B. post.comments.where("content LIKE ?", "%help%").count

C. post.comments.count("content LIKE '%help%'")

D. post.comments.select { |c| c.content == 'help' }.count

Solution

1. Step 1: Use ActiveRecord query for filtering

   To count comments containing 'help', use where with SQL LIKE and then count.

2. Step 2: Analyze each option

   post.comments.where("content LIKE ?", "%help%").count uses correct SQL syntax and chaining. post.comments.count("content LIKE '%help%'") misuses count with a condition string. post.comments.find_all { |c| c.content.include?('help') } loads all comments and filters in Ruby but returns an array, not the count. post.comments.select { |c| c.content == 'help' }.count checks exact match, not substring.

3. Final Answer:

   post.comments.where("content LIKE ?", "%help%").count -> Option B

4. Quick Check:

   Use where with LIKE and count for filtered count [OK]

Hint: Use where with LIKE and count for filtered results [OK]

Common Mistakes:

• Using count with a condition string incorrectly
• Filtering with exact match instead of substring
• Filtering in Ruby instead of database query