Background email delivery in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you call deliver_later on a mailer?

In Rails, when you use deliver_later on a mailer method, what is the expected behavior?

Ruby on Rails

UserMailer.welcome_email(user).deliver_later

A The email is queued to be sent asynchronously by an Active Job worker.

B The email is sent immediately and the method blocks until delivery finishes.

C The email is saved to the database but never sent automatically.

D The email is sent synchronously but only if the server is in development mode.

💡 Hint

Think about how Rails handles background jobs for emails.

Using deliver_later queues the email to be sent asynchronously using Active Job, so the request is not blocked.

📝 Syntax

intermediate

Identify the correct way to enqueue a mailer with arguments

Which of the following is the correct syntax to send a welcome email asynchronously with a user argument?

A UserMailer.deliver_later.welcome_email(user)

B UserMailer.welcome_email(user).deliver_later()

C UserMailer.welcome_email.deliver_later(user)

D UserMailer.welcome_email(user).deliver_now_later

💡 Hint

Remember the order: call mailer method first, then deliver_later.

You call the mailer method with arguments first, then chain deliver_later to enqueue the email.

🔧 Debug

advanced

Why does the email never get sent when using deliver_later?

You have this code in your controller:
UserMailer.welcome_email(@user).deliver_later
But no emails are sent and no errors appear. What is the most likely cause?

A The Active Job queue adapter is set to :test, so jobs are not executed automatically.

B The Active Job queue adapter is set to :inline, so emails are sent immediately.

C The mailer method is missing the <code>mail</code> call inside it.

D The SMTP settings are incorrect, causing silent failures.

💡 Hint

Check your Active Job queue adapter in the environment config.

When the queue adapter is :test, jobs are enqueued but not run automatically, so emails are not sent.

🧠 Conceptual

advanced

What is the benefit of using deliver_later over deliver_now?

Why should you prefer deliver_later instead of deliver_now in a web request?

A deliver_later sends emails synchronously, ensuring immediate delivery.

B deliver_later disables email sending in production to save resources.

C deliver_later stores emails in the database for manual sending later.

D deliver_later queues emails to avoid blocking the web request, improving user experience.

💡 Hint

Think about how web requests should avoid waiting on slow tasks.

deliver_later queues the email to be sent asynchronously, so the user does not wait for the email to send.

❓ state_output

expert

What is the state of the job queue after calling deliver_later once?

Given this code:
UserMailer.notification_email(user).deliver_later
Assuming the queue adapter is :async and no other jobs are running, what is the state of the job queue immediately after this call?

A The job queue is empty because the email is sent immediately.

B The job queue contains one job, but it is delayed by default for 5 minutes.

C The job queue contains one enqueued mail delivery job ready to be processed.

D The job queue contains multiple jobs because deliver_later splits the email into parts.

💡 Hint

deliver_later enqueues exactly one job for the email delivery.

deliver_later enqueues a single job immediately to send the email asynchronously.

Practice

1. What does deliver_later do in Rails email sending?

easy

A. Previews the email without sending it

B. Sends the email immediately and blocks the app until done

C. Schedules the email to be sent at a specific future time

D. Queues the email to be sent in the background without blocking the app

Solution

1. Step 1: Understand deliver_later purpose

   deliver_later queues the email to be sent asynchronously, so the app doesn't wait for the email to finish sending.

2. Step 2: Compare with other options

   deliver_now sends immediately and blocks, scheduling for future time requires extra setup, and previewing doesn't send email.

3. Final Answer:

   Queues the email to be sent in the background without blocking the app -> Option D

4. Quick Check:

   deliver_later = background email sending [OK]

Hint: Remember: deliver_later means send email later in background [OK]

Common Mistakes:

• Confusing deliver_later with deliver_now
• Thinking deliver_later schedules for a specific time
• Assuming deliver_later previews email

2. Which of the following is the correct syntax to send a Rails mailer email in the background?

easy

A. UserMailer.welcome_email.deliver_later(user)

B. UserMailer.welcome_email(user).send_later

C. UserMailer.welcome_email(user).deliver_later

D. UserMailer.deliver_later.welcome_email(user)

Solution

1. Step 1: Recall correct method chaining

   In Rails, you call the mailer method first with parameters, then call deliver_later on the result.

2. Step 2: Check each option's syntax

   UserMailer.welcome_email(user).deliver_later matches the correct pattern: UserMailer.welcome_email(user).deliver_later. Others have incorrect method order or non-existent methods.

3. Final Answer:

   UserMailer.welcome_email(user).deliver_later -> Option C

4. Quick Check:

   Mailer method then deliver_later [OK]

Hint: Call mailer method first, then .deliver_later [OK]

Common Mistakes:

• Swapping method order
• Using non-existent methods like send_later
• Passing parameters after deliver_later

3. Given this code snippet in a Rails app with Active Job configured:

class UserMailer < ApplicationMailer
  def welcome_email(user)
    mail(to: user.email, subject: 'Welcome!')
  end
end

UserMailer.welcome_email(@user).deliver_later

What happens when this code runs?

medium

A. The email is queued to be sent by a background job and the app continues immediately

B. The email is sent immediately and the app waits until it finishes

C. The email is not sent because deliver_later requires extra parameters

D. The code raises an error because deliver_later is not defined

Solution

1. Step 1: Understand deliver_later behavior with Active Job

   When Active Job is set up, calling deliver_later queues the email to be sent asynchronously by a background worker.

2. Step 2: Analyze the code snippet

   The mailer method is called correctly, and deliver_later queues the job. The app does not wait for sending to complete.

3. Final Answer:

   The email is queued to be sent by a background job and the app continues immediately -> Option A

4. Quick Check:

   deliver_later queues email, no blocking [OK]

Hint: deliver_later queues email, app keeps running [OK]

Common Mistakes:

• Thinking deliver_later sends email immediately
• Assuming deliver_later needs extra parameters
• Believing deliver_later is undefined without setup

4. You wrote this code to send an email in the background:

UserMailer.welcome_email(user).deliver_later

But emails are not being sent. What is the most likely cause?

medium

A. You forgot to configure a background job adapter like Sidekiq or Async

B. You should use deliver_now instead of deliver_later

C. The mailer method welcome_email is missing the mail call

D. deliver_later requires a block to work

Solution

1. Step 1: Check background job adapter setup

   deliver_later queues jobs but requires a background job adapter (like Sidekiq, Async, or Delayed Job) to process the queue.

2. Step 2: Evaluate other options

   Using deliver_now sends immediately but blocks. Missing mail call causes no email but not related to background delivery. deliver_later does not require a block.

3. Final Answer:

   You forgot to configure a background job adapter like Sidekiq or Async -> Option A

4. Quick Check:

   Background job adapter needed for deliver_later [OK]

Hint: Check if background job adapter is set up [OK]

Common Mistakes:

• Using deliver_later without job adapter
• Confusing deliver_now with deliver_later
• Thinking deliver_later needs a block

5. You want to send a welcome email only if the user has a confirmed email address, and send it in the background. Which code snippet correctly applies this logic?

hard

A. UserMailer.welcome_email(user).deliver_later unless user.email_confirmed?

B. UserMailer.welcome_email(user).deliver_later if user.email_confirmed?

C. if user.email_confirmed?
UserMailer.welcome_email(user).deliver_now
end

D. UserMailer.welcome_email(user).deliver_later(user.email_confirmed?)

Solution

1. Step 1: Apply conditional check before sending email

   The email should be sent only if user.email_confirmed? is true, so the condition must guard the delivery call.

2. Step 2: Use deliver_later for background sending

   UserMailer.welcome_email(user).deliver_later if user.email_confirmed? correctly uses the condition and calls deliver_later to send asynchronously. if user.email_confirmed? UserMailer.welcome_email(user).deliver_now end sends immediately, not in background. UserMailer.welcome_email(user).deliver_later unless user.email_confirmed? sends when email is not confirmed (wrong logic). UserMailer.welcome_email(user).deliver_later(user.email_confirmed?) passes a boolean as argument incorrectly.

3. Final Answer:

   UserMailer.welcome_email(user).deliver_later if user.email_confirmed? -> Option B

4. Quick Check:

   Condition then deliver_later for background send [OK]

Hint: Check condition before calling deliver_later [OK]

Common Mistakes:

• Sending email immediately instead of background
• Using wrong condition logic
• Passing condition as argument to mailer