Database query optimization in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding N+1 Query Problem in Rails

What is the main cause of the N+1 query problem in Rails ActiveRecord?

A Using eager loading with <code>includes</code> to preload associations

B Loading associated records inside a loop without eager loading

C Querying the database only once for all records

D Using raw SQL queries instead of ActiveRecord methods

💡 Hint

Think about what happens when you access related data inside a loop.

The N+1 query problem happens when Rails loads the main records first, then runs one query per associated record inside a loop, causing many queries. This happens when you don't use eager loading.

❓ component_behavior

intermediate

Effect of Using includes vs joins

Given a Rails model Post with many comments, what is the difference in database queries when using Post.includes(:comments) versus Post.joins(:comments)?

A <code>includes</code> loads only posts; <code>joins</code> loads only comments

B <code>includes</code> performs an INNER JOIN; <code>joins</code> loads posts and comments separately

C <code>includes</code> loads posts and comments in separate queries; <code>joins</code> performs an INNER JOIN returning combined rows

D <code>includes</code> and <code>joins</code> both perform the same single query with LEFT JOIN

💡 Hint

Consider how eager loading and joining affect the number and type of queries.

includes loads the main records and their associations in separate queries to avoid N+1 queries, while joins creates a single query with INNER JOIN combining tables, which can filter results.

📝 Syntax

advanced

Correct Use of select to Optimize Query

Which option correctly uses select in Rails ActiveRecord to fetch only the id and name columns from the User model?

A User.select(:id, :name)

B User.select('id name')

C User.select('id, name')

D User.select(:id name)

💡 Hint

Remember how to pass multiple columns as symbols or strings in select.

The correct syntax is to pass multiple column names as separate symbols or a single string with comma-separated names. Option A uses symbols correctly. Option A uses a string but with a comma, which is valid in SQL and Rails. Option A misses commas. Option A is invalid syntax.

🔧 Debug

advanced

Identifying Cause of Slow Query in Rails

Given this code snippet, why is the query slow?

users = User.all
users.each do |user|
  puts user.profile.bio
end

A Because <code>profile.bio</code> is a method, not a database column

B Because <code>User.all</code> loads too many users at once

C Because <code>puts</code> slows down the loop

D Because <code>profile</code> association is not eager loaded, causing N+1 queries

💡 Hint

Think about how accessing associations inside loops affects queries.

Accessing user.profile inside the loop without eager loading causes one query per user to fetch the profile, leading to many queries and slow performance.

❓ state_output

expert

Output of Query with pluck and distinct

What is the output of this Rails code?

Product.where(category: 'books').pluck(:author).uniq

Assuming the database has authors: ['Alice', 'Bob', 'Alice', 'Carol'] in that category.

A ['Alice', 'Bob', 'Carol']

B ['Alice', 'Bob', 'Alice', 'Carol']

C ['Alice', 'Bob', 'Carol', 'Alice']

D ['Alice', 'Bob']

💡 Hint

Remember that pluck returns an array and uniq is a Ruby method applied after fetching.

pluck fetches the author column values as an array including duplicates. uniq is a Ruby method that removes duplicates from the array. So the output is the array with duplicates removed.