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.