Eager loading (N+1 prevention) in Ruby on Rails - Mini Project: Build & Apply

Eager loading (N+1 prevention) in Rails

📖 Scenario: You are building a simple blog application where each Post has many Comments. You want to display a list of posts along with their comments efficiently.

🎯 Goal: Learn how to use Rails includes method to eager load associated comments for posts, preventing the N+1 query problem.

📋 What You'll Learn

Create a Post model with a has_many :comments association

Create a Comment model with a belongs_to :post association

Write a query to fetch all posts

Add eager loading of comments using includes to prevent N+1 queries

Display posts with their comments in a view

💡 Why This Matters

🌍 Real World

Eager loading is used in web applications to efficiently load related data and avoid slow database queries that happen when loading associations one by one.

💼 Career

Understanding and preventing N+1 queries is a key skill for Rails developers to build fast and scalable applications.

1

Set up Post and Comment models with associations

Create a Post model with has_many :comments and a Comment model with belongs_to :post associations.

Ruby on Rails

# Add Post and Comment models with associations here

Hint

Use has_many in Post and belongs_to in Comment to set up associations.

2

Fetch all posts without eager loading

Write a query to fetch all posts and assign it to a variable called posts using Post.all.

Ruby on Rails

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

# Fetch all posts
# Your code here

Hint

Use Post.all to get all posts and assign to posts.

3

Add eager loading of comments to prevent N+1 queries

Modify the query to fetch all posts with their comments eagerly loaded using includes(:comments) and assign it to posts.

Ruby on Rails

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

# Fetch all posts with comments eager loaded
# Your code here

Hint

Use Post.includes(:comments).all to eager load comments with posts.

4

Display posts with their comments in a view

In a Rails view, use a each loop to iterate over posts and display each post's title and its comments' content.

Ruby on Rails

  
  # Your code here

Hint

Use nested each loops to show posts and their comments.

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