Eager loading (N+1 prevention) in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to eager load the associated posts for users.

Ruby on Rails

users = User.[1](:posts)

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

Awhere

Bjoins

Cselect

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'joins' instead of 'includes' causes N+1 queries.

Using 'select' does not load associations.

Using includes loads the associated posts in one query to prevent N+1 queries.

2fill in blank

medium

Complete the code to eager load comments for posts.

Ruby on Rails

posts = Post.[1](:comments)

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

Aincludes

Bwhere

Corder

Dgroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'where' filters records but does not eager load.

Using 'order' or 'group' does not load associations.

includes loads the comments with posts to prevent multiple queries.

3fill in blank

hard

Fix the error in the code to eager load authors for articles.

Ruby on Rails

articles = Article.[1](:author)

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

Aincludes

Bpluck

Cfind

Dload

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'load' or 'find' does not eager load associations.

Using 'pluck' only selects columns, no associations.

includes is the correct method to eager load the author association.

4fill in blank

hard

Fill both blanks to eager load comments and their authors for posts.

Ruby on Rails

posts = Post.[1]([2]: :author)

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

Aincludes

Bjoins

Ccomments

Dwhere

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'joins' instead of 'includes' causes N+1 queries.

Using 'where' does not eager load associations.

Use includes with nested association comments: :author to eager load both.

5fill in blank

hard

Fill all three blanks to eager load categories, tags, and their creators for articles.

Ruby on Rails

articles = Article.[1]([2], [3]: :creator)

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

Aincludes

Bcategories

Ctags

Djoins

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'joins' instead of 'includes' causes multiple queries.

Not nesting the creator association under tags.

Use includes with :categories and nested :tags => :creator to eager load all associations.

Practice

1. What is the main purpose of using includes in Rails when querying data?

easy

A. To load associated records in fewer database queries and avoid N+1 problems

B. To delete associated records automatically

C. To sort records alphabetically

D. To create new associated records

Solution

1. Step 1: Understand N+1 query problem

   When loading records with associations, Rails may run one query for the main records and one query per associated record, causing many queries.

2. Step 2: Role of includes

   The includes method tells Rails to load all associated records in one or few queries, preventing many extra queries.

3. Final Answer:

   To load associated records in fewer database queries and avoid N+1 problems -> Option A

4. Quick Check:

   Eager loading = fewer queries = better speed [OK]

Hint: Remember: includes = fewer queries, faster loading [OK]

Common Mistakes:

• Thinking includes deletes records
• Confusing includes with sorting
• Assuming includes creates new records

2. Which of the following is the correct syntax to eager load the comments association for Post records?

easy

A. Post.eager_load(:comments)

B. Post.load_comments

C. Post.includes(:comments)

D. Post.with_comments

Solution

1. Step 1: Identify the standard eager loading method

   Rails uses includes to eager load associations simply and efficiently.

2. Step 2: Check syntax correctness

   Post.includes(:comments) is the correct syntax to load posts with their comments in fewer queries.

3. Final Answer:

   Post.includes(:comments) -> Option C

4. Quick Check:

   Correct eager loading syntax = includes [OK]

Hint: Use includes(:association) to eager load in Rails [OK]

Common Mistakes:

• Using non-existent methods like load_comments
• Confusing eager_load with includes (different behavior)
• Using with_comments which is not a Rails method

3. Given the code below, what will be the total number of database queries executed?

posts = Post.all
posts.each do |post|
  puts post.comments.count
end

medium

A. 1 query

B. N+1 queries, where N is the number of posts

C. 2 queries

D. No queries

Solution

1. Step 1: Analyze the code without eager loading

   Post.all loads all posts in 1 query. Then inside the loop, post.comments.count triggers a query per post to count comments.

2. Step 2: Calculate total queries

   1 query for posts + N queries for comments count = N+1 queries total.

3. Final Answer:

   N+1 queries, where N is the number of posts -> Option B

4. Quick Check:

   Without includes, each association call triggers a query [OK]

Hint: Looping over associations without includes causes N+1 queries [OK]

Common Mistakes:

• Assuming only 1 query runs
• Thinking count does not query database
• Confusing eager_load with includes

4. You wrote this code to eager load comments but still see many queries:

posts = Post.includes(:comments).where(published: true)
posts.each do |post|
  puts post.comments.count
end

What is the likely cause of the N+1 queries still happening?

medium

A. Eager loading only works with all method

B. The where clause disables eager loading

C. You must use joins instead of includes

D. Using count on association triggers a separate query despite eager loading

Solution

1. Step 1: Understand how count works on associations

   Calling post.comments.count runs a SQL COUNT query for each post, ignoring preloaded records.

2. Step 2: Why eager loading doesn't prevent count queries

   includes loads all comments, but count asks database for count separately, causing N+1 queries.

3. Final Answer:

   Using count on association triggers a separate query despite eager loading -> Option D

4. Quick Check:

   count triggers query even with includes [OK]

Hint: Use size or length on loaded associations to avoid extra queries [OK]

Common Mistakes:

• Thinking where disables includes
• Believing joins always eager load
• Assuming eager loading needs all method

5. You want to display a list of authors with their books and each book's reviews efficiently. Which is the best way to eager load these nested associations in Rails?

hard

A. Author.includes(books: :reviews)

B. Author.includes(:books, :reviews)

C. Author.joins(:books).joins(:reviews)

D. Author.eager_load(:books, :reviews)

Solution

1. Step 1: Understand nested associations eager loading

   To load authors with books and each book's reviews, nested includes syntax is needed.

2. Step 2: Choose correct nested includes syntax

   Author.includes(books: :reviews) loads authors, their books, and reviews in minimal queries.

3. Step 3: Why other options are incorrect

   Author.includes(:books, :reviews) tries to load reviews directly on authors (wrong association). Author.joins(:books).joins(:reviews) uses joins which may not eager load. Author.eager_load(:books, :reviews) eager_load loads with joins but is less flexible.

4. Final Answer:

   Author.includes(books: :reviews) -> Option A

5. Quick Check:

   Nested includes = includes(parent: :child) [OK]

Hint: Use nested hash in includes for nested associations [OK]

Common Mistakes:

• Trying to include unrelated associations directly
• Using joins instead of includes for eager loading
• Misunderstanding nested includes syntax