Eager loading (N+1 prevention) in Ruby on Rails - Performance & Optimization

Performance: Eager loading (N+1 prevention)

HIGH IMPACT

This concept affects database query efficiency and page load speed by reducing the number of queries needed to fetch related data.

Loading a list of posts with their authors

Ruby on Rails

posts = Post.includes(:author).all
posts.each do |post|
  puts post.author.name
end

Eager loading fetches posts and their authors in two queries regardless of number of posts, reducing database hits.

📈 Performance GainTriggers only 2 database queries, significantly reducing load time and server work.

Loading a list of posts with their authors

Ruby on Rails

posts = Post.all
posts.each do |post|
  puts post.author.name
end

This triggers one query for posts plus one query per post to fetch the author, causing many database hits (N+1 queries).

📉 Performance CostTriggers 1 + N database queries, increasing page load time and server load.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
N+1 queries (lazy loading)	Normal	Multiple due to delayed data	Higher due to slower data	[X] Bad
Eager loading with includes	Normal	Single or fewer	Lower due to faster data	[OK] Good

Rendering Pipeline

Eager loading reduces the number of database queries, which speeds up data retrieval before rendering. This leads to faster style calculation and layout since data is ready sooner.

→Data Fetching

→Layout

→Paint

⚠️ BottleneckData Fetching (database queries)

Core Web Vital Affected

LCP

This concept affects database query efficiency and page load speed by reducing the number of queries needed to fetch related data.

Optimization Tips

1Avoid N+1 queries by using eager loading with includes or preload.

2Batch related data fetching to reduce database round trips.

3Monitor database queries in DevTools Network tab to detect N+1 issues.

Performance Quiz - 3 Questions

What is the main performance problem caused by N+1 queries?

A Many small database queries increasing load time

B Too much CSS causing slow paint

C Large JavaScript bundles blocking rendering

D Excessive DOM nodes causing reflows

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by XHR or fetch, reload page, count number of API/database calls made.

What to look for: Look for multiple repeated queries fetching related data separately (bad) versus fewer batched queries (good).

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