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.

Practice

1. What is the main benefit of using includes in Rails queries?

easy

A. It loads associated records in fewer database queries to improve performance.

B. It deletes unused records from the database automatically.

C. It encrypts data before saving to the database.

D. It creates new database tables for associations.

Solution

1. Step 1: Understand what includes does

   includes loads related records in one or fewer queries to avoid repeated database hits.

2. Step 2: Identify the benefit

   This reduces the number of queries, speeding up the app and saving server resources.

3. Final Answer:

   It loads associated records in fewer database queries to improve performance. -> Option A

4. Quick Check:

   Using includes reduces queries = B [OK]

Hint: Think: fewer queries means faster loading [OK]

Common Mistakes:

• Confusing includes with deleting records
• Thinking includes encrypts data
• Assuming includes creates tables

2. Which of the following is the correct syntax to select only the name and email columns from a User model in Rails?

easy

A. User.find(:name, :email)

B. User.where(:name, :email)

C. User.includes(:name, :email)

D. User.select(:name, :email)

Solution

1. Step 1: Recall the method to choose specific columns

   The select method specifies which columns to retrieve from the database.

2. Step 2: Check the syntax

   User.select(:name, :email) correctly selects only the name and email columns.

3. Final Answer:

   User.select(:name, :email) -> Option D

4. Quick Check:

   Select columns = select method = D [OK]

Hint: Use select to pick columns, not where or includes [OK]

Common Mistakes:

• Using where instead of select for columns
• Using includes to select columns
• Using find with multiple columns

3. Given the following Rails code:

posts = Post.includes(:comments).where(published: true)

What is the main effect of this query?

medium

A. It loads all posts and their comments, regardless of published status.

B. It loads only comments for published posts but not the posts themselves.

C. It loads only published posts and their comments efficiently in fewer queries.

D. It deletes unpublished posts and their comments.

Solution

1. Step 1: Analyze the query filters and includes

   The where(published: true) filters posts to only those published. includes(:comments) loads comments with posts efficiently.

2. Step 2: Understand the combined effect

   The query fetches published posts and their comments in fewer database queries, improving performance.

3. Final Answer:

   It loads only published posts and their comments efficiently in fewer queries. -> Option C

4. Quick Check:

   Filter + includes = efficient published posts with comments = C [OK]

Hint: Filter first, then includes loads related data efficiently [OK]

Common Mistakes:

• Thinking includes ignores the where filter
• Assuming comments load without posts
• Confusing includes with deletion

4. Identify the problem in this Rails query and how to fix it:

users = User.where(active: true).includes(:posts).select(:name)

medium

A. The select excludes the primary key (:id), causing posts not to load; fix by including :id in select.

B. The includes method is used incorrectly; replace it with joins.

C. The where clause should come after select to filter correctly.

D. The query will delete inactive users; remove where clause.

Solution

1. Step 1: Understand select limits columns

   The select(:name) limits columns returned, but the primary key id is missing.

2. Step 2: Effect on includes

   Without the primary key, Rails cannot load associated posts properly with includes.

3. Step 3: Fix the query

   Add the primary key column (e.g., :id) to the select list to allow loading posts.

4. Final Answer:

   The select excludes the primary key (:id), causing posts not to load; fix by including :id in select. -> Option A

5. Quick Check:

   Select must include primary key for includes to work = A [OK]

Hint: Always select primary keys when using includes [OK]

Common Mistakes:

• Removing primary key from select breaks associations
• Replacing includes with joins unnecessarily
• Changing where order incorrectly

5. You want to optimize a query to fetch Order records with their Customer and only the total_price and created_at fields from orders. Which Rails query is best for this optimization?

hard

A. Order.joins(:customer).select('orders.total_price, orders.created_at, customers.*')

B. Order.includes(:customer).select(:total_price, :created_at, :customer_id)

C. Order.includes(:customer).select(:total_price, :created_at)

D. Order.all.select(:total_price, :created_at)

Solution

1. Step 1: Identify needed fields and associations

   You want orders with only total_price and created_at, plus associated customers.

2. Step 2: Ensure foreign key is selected for includes

   Using includes(:customer) requires selecting the foreign key customer_id to load customers efficiently.

3. Step 3: Choose the query that selects needed fields and includes association correctly

   Order.includes(:customer).select(:total_price, :created_at, :customer_id) meets all requirements.

4. Final Answer:

   Order.includes(:customer).select(:total_price, :created_at, :customer_id) -> Option B

5. Quick Check:

   Select needed fields + foreign key + includes = A [OK]

Hint: Include foreign keys in select when using includes [OK]

Common Mistakes:

• Omitting foreign key breaks includes
• Using joins loads all customer fields unnecessarily
• Selecting all orders without includes