Eager loading (N+1 prevention) in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output count of queries executed?

Given the following Rails code, how many SQL queries will be executed when rendering the posts and their comments count?

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

Ruby on Rails

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

A 1 + N queries (N = number of posts)

B 1 query

C N queries

D 2 queries

💡 Hint

Think about how Rails loads associations by default.

Without eager loading, Rails loads posts in one query, then loads comments count for each post separately, causing N additional queries.

📝 Syntax

intermediate

Which code snippet correctly eager loads comments for posts?

Select the Rails code that correctly eager loads comments to prevent N+1 queries when accessing post.comments.

A posts = Post.preload(:comments).where(comments: { approved: true })

B posts = Post.joins(:comments).all

C posts = Post.includes(:comments).all

D posts = Post.eager_load(:comments).where('comments.created_at > ?', 1.week.ago)

💡 Hint

Look for the method that loads associations in one query or minimal queries.

includes loads associated comments efficiently to avoid N+1 queries. joins only joins tables but does not eager load. preload loads separately but does not filter well. eager_load forces LEFT OUTER JOIN and can cause complex queries.

🔧 Debug

advanced

Why does this eager loading code still cause N+1 queries?

Consider this code snippet:

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

Why does it still cause multiple queries?

Ruby on Rails

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

A Because filtering with where on association triggers a new query per post

B Because includes does not load comments at all

C Because count always triggers a single query for all posts

D Because posts is not loaded before iteration

💡 Hint

Think about how where on an association affects eager loading.

Using where on the association inside the loop triggers a new query per post, causing N+1 despite includes.

🧠 Conceptual

advanced

What is the main difference between includes and preload in Rails?

Choose the statement that best describes the difference between includes and preload methods for eager loading associations.

A includes only works with belongs_to, preload only with has_many

B includes always loads associations in separate queries, preload uses JOIN

C includes and preload are identical in behavior and performance

D includes uses LEFT OUTER JOIN, preload loads associations in separate queries

💡 Hint

Think about how each method loads associated records.

includes can use JOIN or separate queries depending on usage, preload always uses separate queries.

❓ state_output

expert

What is the output of this Rails console code regarding query count?

Given the following code in Rails console, what will be the total number of SQL queries executed?

ActiveRecord::Base.logger = Logger.new(STDOUT)
posts = Post.eager_load(:comments).where(comments: { approved: true }).distinct
posts.each do |post|
  puts post.comments.size
end

Ruby on Rails

ActiveRecord::Base.logger = Logger.new(STDOUT)
posts = Post.eager_load(:comments).where(comments: { approved: true }).distinct
posts.each do |post|
  puts post.comments.size
end

A 2 queries

B 1 query

C N + 1 queries

D No queries

💡 Hint

eager_load uses JOIN and loads all data in one query.

eager_load uses LEFT OUTER JOIN and loads posts with filtered comments in one query. Accessing comments.size does not trigger extra queries.