In Stock Email Notifications

Action Mailer is a feature of Rails that allows you to send emails. We’ll use it to notify subscribers when a product is back in stock.

We can generate a mailer with the following command:

Terminal window
$ bin/rails g mailer Product in_stock

This generates a class at with an in_stock method.

Update this method to mail to a subscriber’s email address.

class ProductMailer < ApplicationMailer
  # Subject can be set in your I18n file at config/locales/en.yml
  # with the following lookup:
  #
  #   en.product_mailer.in_stock.subject
  #
  def in_stock
    @product = params[:product]
    mail to: params[:subscriber].email
  end
end

The mailer generator also generates two email templates in our views folder: one for HTML and one for Text. We can update those to include a message and link to the product.

Change to:

<h1>Good news!</h1>


<p><%= link_to @product.name, product_url(@product) %> is back in stock.</p>

And to:

Good news!


<%= @product.name %> is back in stock.
<%= product_url(@product) %>

We use product_url instead of product_path in mailers because email clients need to know the full URL to open in the browser when the link is clicked.

We can test an email by opening the Rails console and loading a product and subscriber to send to:

store(dev)> product = Product.first
store(dev)> subscriber = product.subscribers.find_or_create_by(email: "subscriber@example.org")
store(dev)> ProductMailer.with(product: product, subscriber: subscriber).in_stock.deliver_later

You’ll see that it prints out an email in the logs.

ProductMailer#in_stock: processed outbound mail in 63.0ms
Delivered mail 66a3a9afd5d4a_108b04a4c41443@local.mail (33.1ms)
Date: Fri, 26 Jul 2024 08:50:39 -0500
From: from@example.com
To: subscriber@example.com
Message-ID: <66a3a9afd5d4a_108b04a4c41443@local.mail>
Subject: In stock
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_66a3a9afd235e_108b04a4c4136f";
 charset=UTF-8
Content-Transfer-Encoding: 7bit




----==_mimepart_66a3a9afd235e_108b04a4c4136f
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit


Good news!


T-Shirt is back in stock.
http://localhost:3000/products/1




----==_mimepart_66a3a9afd235e_108b04a4c4136f
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: 7bit


<!-- BEGIN app/views/layouts/mailer.html.erb --><!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <style>
      /* Email styles need to be inline */
    </style>
  </head>


  <body>
    <!-- BEGIN app/views/product_mailer/in_stock.html.erb --><h1>Good news!</h1>


<p><a href="http://localhost:3000/products/1">T-Shirt</a> is back in stock.</p>
<!-- END app/views/product_mailer/in_stock.html.erb -->
  </body>
</html>
<!-- END app/views/layouts/mailer.html.erb -->
----==_mimepart_66a3a9afd235e_108b04a4c4136f--


Performed ActionMailer::MailDeliveryJob (Job ID: 5e2bd5f2-f54f-4088-ace3-3f6eb15aaf46) from Async(default) in 111.34ms

To trigger these emails, we can use a callback in the Product model to send emails anytime the inventory count changes from 0 to a positive number.

class Product < ApplicationRecord
  has_many :subscribers, dependent: :destroy
  has_one_attached :featured_image
  has_rich_text :description


  validates :name, presence: true
  validates :inventory_count, numericality: { greater_than_or_equal_to: 0 }


  after_update_commit :notify_subscribers, if: :back_in_stock?


  def back_in_stock?
    inventory_count_previously_was&.zero? && inventory_count > 0
  end


  def notify_subscribers
    subscribers.each do |subscriber|
      ProductMailer.with(product: self, subscriber: subscriber).in_stock.deliver_later
    end
  end
end

after_update_commit is an Active Record callback that is fired after changes are saved to the database. if: :back_in_stock? tells the callback to run only if the back_in_stock? method returns true.

Active Record keeps track of changes to attributes so back_in_stock? checks the previous value of inventory_count using inventory_count_previously_was. Then we can compare that against the current inventory count to determine if the product is back in stock.

notify_subscribers uses the Active Record association to query the subscribers table for all subscribers for this specific product and then queues up the in_stock email to be sent to each of them.

Adding Subscribers to Products
Extracting a Concern
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