has_many relationship in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a has_many relationship in a Rails model.

Ruby on Rails

class Author < ApplicationRecord
  [1] :books
end

Drag options to blanks, or click blank then click option'

Ainclude

Bbelongs_to

Chas_one

Dhas_many

Attempts:

3 left

💡 Hint

Common Mistakes

Using belongs_to instead of has_many

Using has_one which is for one-to-one relationships

The has_many method declares a one-to-many relationship from Author to Book models.

2fill in blank

medium

Complete the code to define the inverse belongs_to relationship in the associated model.

Ruby on Rails

class Book < ApplicationRecord
  [1] :author
end

Drag options to blanks, or click blank then click option'

Abelongs_to

Bhas_many

Chas_one

Dinclude

Attempts:

3 left

💡 Hint

Common Mistakes

Using has_many instead of belongs_to

Forgetting to define the inverse relationship

The belongs_to method sets the many-to-one side of the relationship from Book to Author.

3fill in blank

hard

Fix the error in the has_many declaration to specify dependent destroy behavior.

Ruby on Rails

class Author < ApplicationRecord
  has_many :books, [1]: :destroy
end

Drag options to blanks, or click blank then click option'

Adependent

Bdelete_all

Cdestroy_all

Ddependent_destroy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'dependent_destroy' which is not a valid option

Using 'destroy_all' which is a method, not an option

The dependent: :destroy option ensures that when an author is deleted, their books are also deleted.

4fill in blank

hard

Fill both blanks to create a has_many through association between Author and Genre through BookGenre.

Ruby on Rails

class Author < ApplicationRecord
  has_many :book_genres
  has_many :[1], through: :[2]
end

Drag options to blanks, or click blank then click option'

Agenres

Bbooks

Cbook_genres

Dauthors

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the join model and target model names

Using singular names instead of plural

The has_many :genres, through: :book_genres sets up a many-to-many relationship through the join model BookGenre.

5fill in blank

hard

Fill all three blanks to define a has_many association with a custom foreign key and class name.

Ruby on Rails

class User < ApplicationRecord
  has_many :[1], foreign_key: '[2]', class_name: '[3]'
end

Drag options to blanks, or click blank then click option'

Aauthored_books

Bauthor_id

CBook

Dbooks

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong foreign key name

Not matching the class_name to the actual model

This code defines a has_many association named authored_books that uses author_id as the foreign key and points to the Book model.

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