Email previews in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does Rails email preview render the email?

In Rails, when you open an email preview in the browser, what does Rails do to show the email content?

A It calls the mailer method and renders the email body as HTML or text in the browser.

B It sends the email to the configured SMTP server and shows the server response.

C It generates a PDF of the email and displays it in the browser.

D It only shows the email subject line without rendering the body.

💡 Hint

Think about how you can see the full email content without sending it.

Rails email previews call the mailer method and render the email body directly in the browser. This lets you see the email as it would appear to the user without actually sending it.

📝 Syntax

intermediate

Correct syntax to define a preview class for UserMailer

Which of the following is the correct way to define a preview class for a mailer named UserMailer in Rails?

A

class UserMailerPreview < ActionMailer::Preview
  def welcome_email
    UserMailer.welcome_email(User.first)
  end
end

B

class UserMailerPreview
  def welcome_email
    UserMailer.welcome_email(User.first)
  end
end

C

class UserMailerPreview < ApplicationMailer
  def welcome_email
    UserMailer.welcome_email(User.first)
  end
end

D

class UserMailerPreview < ActionController::Base
  def welcome_email
    UserMailer.welcome_email(User.first)
  end
end

💡 Hint

Preview classes inherit from a special Rails class for previews.

Preview classes must inherit from ActionMailer::Preview to work correctly in Rails.

❓ state_output

advanced

What is the output of this preview method?

Given this preview method in Rails, what will be the subject line shown in the preview if User.first has email '[e-mail]'? class UserMailerPreview < ActionMailer::Preview def notify UserMailer.notify(User.first) end end And the mailer method: class UserMailer < ApplicationMailer def notify(user) mail(to: user.email, subject: "Hello, #{user.email}") end end

A Hello, [e-mail]

B No subject line

C Hello, [e-mail]

D Hello,

💡 Hint

Look at how the subject is built using the user email.

The subject line uses string interpolation to include the user's email, so it will show "Hello, [e-mail]".

🔧 Debug

advanced

Why does this email preview raise an error?

This preview method raises an error when accessed: class UserMailerPreview < ActionMailer::Preview def alert UserMailer.alert(nil) end end Mailer method: class UserMailer < ApplicationMailer def alert(user) mail(to: user.email, subject: "Alert") end end What is the cause of the error?

A Preview class must inherit from ApplicationMailer.

B Calling user.email on nil causes a NoMethodError.

C Mailer method alert is not defined properly.

D The mail method requires a block to render the email.

💡 Hint

Check what happens when you call a method on nil.

Passing nil as user causes user.email to raise a NoMethodError because nil has no email method.

🧠 Conceptual

expert

Why use email previews in Rails development?

Which of the following best explains the main benefit of using email previews in Rails development?

A They generate email templates in multiple languages automatically.

B They automatically send test emails to all users to verify email delivery.

C They replace the need for writing mailer tests in the test suite.

D They let developers see the email content in the browser without sending actual emails, speeding up design and testing.

💡 Hint

Think about how previews help before sending emails.

Email previews allow developers to view and verify email content quickly in the browser without sending real emails, making design and testing faster and safer.

Practice

1. What is the main purpose of email previews in Rails?

easy

A. To view the email content in the browser before sending

B. To send emails automatically to users

C. To store emails in the database

D. To encrypt email content for security

Solution

1. Step 1: Understand the role of email previews

   Email previews allow developers to see how emails will look before sending them out.

2. Step 2: Compare options with this purpose

   Only To view the email content in the browser before sending describes viewing email content in the browser, which matches the purpose of previews.

3. Final Answer:

   To view the email content in the browser before sending -> Option A

4. Quick Check:

   Email previews = View email before sending [OK]

Hint: Email previews show email content before sending [OK]

Common Mistakes:

• Confusing previews with sending emails
• Thinking previews store emails permanently
• Assuming previews encrypt emails

2. Which of the following is the correct way to define a preview method in a Rails mailer preview class?

easy

A. def welcome_email; UserMailer.welcome_email(User.first); end

B. def welcome_email() UserMailer.welcome_email(User.first) end

C. def welcome_email { UserMailer.welcome_email(User.first) }

D. def welcome_email -> UserMailer.welcome_email(User.first)

Solution

1. Step 1: Recall Ruby method syntax in classes

   Methods are defined with def method_name; ...; end syntax.

2. Step 2: Check each option's syntax

   def welcome_email; UserMailer.welcome_email(User.first); end uses correct Ruby method syntax with semicolons and end. Options A, C, and D have syntax errors or invalid Ruby syntax.

3. Final Answer:

   def welcome_email; UserMailer.welcome_email(User.first); end -> Option A

4. Quick Check:

   Correct Ruby method syntax = def welcome_email; UserMailer.welcome_email(User.first); end [OK]

Hint: Use def ...; ...; end for methods in Ruby classes [OK]

Common Mistakes:

• Omitting 'end' keyword
• Using braces {} instead of def/end
• Using arrow syntax which is invalid here

3. Given this preview method in UserMailerPreview:

def notification_email
  UserMailer.notification_email(User.find(1))
end

What will happen when you visit /rails/mailers/user_mailer/notification_email in the browser?

medium

A. An error occurs because UserMailerPreview is missing

B. The email content for user with ID 1 is displayed in the browser

C. The email is sent to user with ID 1 automatically

D. The browser shows a 404 Not Found error

Solution

1. Step 1: Understand what preview methods do

   Preview methods return mail objects that Rails renders as HTML in the browser for preview.

2. Step 2: Analyze the URL and method

   The URL matches the preview method name and class, so Rails will render the email content for user ID 1.

3. Final Answer:

   The email content for user with ID 1 is displayed in the browser -> Option B

4. Quick Check:

   Preview URL shows email content = The email content for user with ID 1 is displayed in the browser [OK]

Hint: Preview URL shows email content, not send email [OK]

Common Mistakes:

• Thinking preview sends email
• Assuming missing preview class error
• Expecting 404 if preview is defined

4. You wrote this preview method:

def alert_email
  UserMailer.alert_email(user)
end

But visiting the preview URL gives an error: undefined local variable or method `user'. What is the fix?

medium

A. Add @user as an instance variable outside the method

B. Change method name to user_alert_email

C. Remove the method and call UserMailer directly

D. Define the variable user inside the method before calling UserMailer

Solution

1. Step 1: Identify cause of error

   The error says user is undefined, meaning it was never assigned a value.

2. Step 2: Fix by defining user variable

   Inside the method, assign user = User.first or similar before calling UserMailer.alert_email(user).

3. Final Answer:

   Define the variable user inside the method before calling UserMailer -> Option D

4. Quick Check:

   Undefined variable error = define variable first [OK]

Hint: Always define variables before use in preview methods [OK]

Common Mistakes:

• Assuming method name causes error
• Trying to call mailer without arguments
• Using instance variable outside method scope

5. You want to preview an email that requires a list of users. Which is the best way to write the preview method in UserMailerPreview?

hard

A. def newsletter_email
UserMailer.newsletter_email(User.all)
end

B. def newsletter_email(users)
UserMailer.newsletter_email(users)
end

C. def newsletter_email
users = User.limit(3)
UserMailer.newsletter_email(users)
end

D. def newsletter_email
UserMailer.newsletter_email
end

Solution

1. Step 1: Understand preview method context

   Preview methods do not accept parameters; they must define data inside the method.

2. Step 2: Evaluate options for passing user list

   def newsletter_email users = User.limit(3) UserMailer.newsletter_email(users) end defines a limited user list inside the method and passes it correctly. def newsletter_email(users) UserMailer.newsletter_email(users) end incorrectly defines a parameter. def newsletter_email UserMailer.newsletter_email(User.all) end passes all users which may be too large. def newsletter_email UserMailer.newsletter_email end passes no argument, likely causing error.

3. Final Answer:

   def newsletter_email users = User.limit(3) UserMailer.newsletter_email(users) end -> Option C

4. Quick Check:

   Define data inside preview method, no parameters [OK]

Hint: Preview methods must define data inside, no parameters [OK]

Common Mistakes:

• Adding parameters to preview methods
• Passing no arguments when required
• Using too large datasets in previews