N+1 detection tools in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Detecting N+1 queries with Bullet gem

Given a Rails app using the Bullet gem, what will Bullet report when rendering a view that loads Post records and then accesses each post's comments without eager loading?

Ruby on Rails

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

A Bullet will report a syntax error due to missing eager loading.

B Bullet will not report any warnings because Post.all loads all posts at once.

C Bullet will report an N+1 query warning because comments are accessed without eager loading.

D Bullet will report a performance warning unrelated to N+1 queries.

💡 Hint

Think about what happens when you access an association inside a loop without eager loading.

Bullet detects when your code triggers multiple database queries inside loops due to missing eager loading. Here, accessing post.comments inside the loop causes an N+1 query problem.

📝 Syntax

intermediate

Correct usage of includes to avoid N+1

Which option correctly uses includes to avoid N+1 queries when loading posts and their comments?

Ruby on Rails

posts = Post.???
posts.each do |post|
  post.comments.each do |comment|
    puts comment.body
  end
end

A select(:comments)

B includes(:comments)

C preload(:comments).where('comments.count > 0')

D joins(:comments)

💡 Hint

Which method loads associated records to prevent extra queries?

includes(:comments) loads posts and their comments in two queries, preventing N+1. joins only joins tables but does not eager load. select changes selected columns. preload is valid but the added where clause is invalid here.

🔧 Debug

advanced

Identifying cause of false positive N+1 detection

Using the Bullet gem, you get an N+1 warning on user.profile even though you used includes(:profile). What is the most likely cause?

Ruby on Rails

users = User.includes(:profile).where(active: true)
users.each do |user|
  puts user.profile.bio
end

A The <code>profile</code> association uses a custom SQL that disables eager loading.

B You forgot to call <code>references(:profile)</code> along with <code>includes</code>.

C The <code>profile</code> association is defined with <code>optional: true</code>, causing Bullet to misdetect.

D Bullet cannot detect N+1 queries on <code>has_one</code> associations.

💡 Hint

Think about how custom SQL in associations affects eager loading.

If the association uses a custom SQL query or scope that disables eager loading, Bullet may report false positives because the data is not preloaded as expected.

🧠 Conceptual

advanced

Understanding Skylight's N+1 detection approach

How does Skylight detect N+1 queries in a Rails application?

A By scanning the source code for missing <code>includes</code> calls in ActiveRecord queries.

B By monitoring CPU usage spikes during database calls.

C By requiring manual annotations in the code to mark potential N+1 spots.

D By analyzing the number of similar SQL queries executed during a single request and flagging repeated queries on associations.

💡 Hint

Think about how a tool can detect repeated queries dynamically.

Skylight tracks SQL queries during requests and identifies patterns where many similar queries are executed repeatedly, indicating N+1 problems.

❓ state_output

expert

Output of Rack Mini Profiler on N+1 queries

Given this Rails controller code, what output will Rack Mini Profiler show regarding database queries?

def index
  @users = User.all
  @users.each do |user|
    user.posts.each do |post|
      puts post.title
    end
  end
end

A It will show 1 query for users and multiple queries for posts, indicating an N+1 problem.

B It will show a single optimized query joining users and posts.

C It will show no database queries because data is cached.

D It will show a syntax error in the controller code.

💡 Hint

Consider how accessing associations inside loops affects query count.

Without eager loading, accessing user.posts inside a loop triggers one query per user, causing multiple queries and an N+1 problem visible in Rack Mini Profiler.