Email previews in Ruby on Rails - Step-by-Step Execution

Concept Flow - Email previews

Write Mailer Method

↓

Create Preview Class

↓

Start Rails Server

↓

Access Preview URL

↓

View Email Rendered in Browser

↓

Test & Adjust Email Content

↓

Use Preview for Development

This flow shows how you write a mailer, create a preview class, run the server, and view the email in the browser for easy testing.

Execution Sample

Ruby on Rails

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

Defines a mailer method that sends a welcome email to a user.

Execution Table

Step	Action	Input	Output	Notes
1	Define mailer method	user object	Mailer method ready	UserMailer#welcome_email defined
2	Create preview class	UserMailer	Preview class ready	UserMailerPreview with welcome_email method
3	Start Rails server	rails server	Server running	Previews accessible at /rails/mailers
4	Access preview URL	/rails/mailers/user_mailer/welcome_email	Email rendered in browser	Shows email with user data
5	Adjust email content	Edit mailer or view	Updated preview	Changes reflected immediately
6	Use preview for testing	View in browser	Visual check of email	No need to send real emails
7	Exit	Stop server	Previews unavailable	Development ends

💡 Rails server stops or preview URL not accessed, so email previews are no longer shown.

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
user	nil	User instance with email	User instance passed to preview	User instance used in email
@user	nil	Set to user in mailer	Used in preview rendering	Used in email template
mail	nil	Mail object created	Rendered as HTML in browser	Displayed in preview

Key Moments - 3 Insights

Why do we need a preview class separate from the mailer?

The preview class lets Rails render the email in the browser without sending it. See execution_table step 2 and 4 where the preview class is created and used to show the email.

How does Rails know which email to show in the preview URL?

The URL matches the mailer and method names, like /rails/mailers/user_mailer/welcome_email, which Rails maps to the preview method. See execution_table step 4.

Can I see changes immediately after editing the mailer or views?

Yes, because the preview reloads on each request in development. See execution_table step 5 where changes update the preview instantly.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 4?

A The Rails server starts running

B The mailer method is defined

C The email is rendered in the browser preview

D The preview class is created

💡 Hint

Check the 'Output' column in execution_table row 4

At which step does the preview class get created?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look at the 'Action' column in execution_table row 2

If you stop the Rails server, what happens to the email previews?

A They continue to show in the browser

B They show an error or are unavailable

C They send real emails automatically

D They cache and show old previews

💡 Hint

See the exit_note and execution_table step 7

Concept Snapshot

Email previews in Rails let you see emails in the browser without sending them.
Create a preview class matching your mailer.
Start the Rails server and visit /rails/mailers.
Edit mailer or views to update previews instantly.
Use previews for easy, safe email testing during development.

Full Transcript

Email previews in Rails help developers see how emails look without sending them. First, you write a mailer method that defines the email content. Then, you create a preview class that calls this mailer method with sample data. When you start the Rails server, you can visit a special URL like /rails/mailers/user_mailer/welcome_email to see the email rendered in your browser. This lets you check the design and content quickly. If you change the mailer or its views, the preview updates immediately. When you stop the server, previews are no longer available. This process saves time and avoids sending test emails to real users.

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