Extracting a Concern

The Product model now has a decent amount of code for handling notifications. To better organize our code, we can extract this to an ActiveSupport::Concern. A Concern is a Ruby module with some syntactic sugar to make using them easier.

First let’s create the Notifications module.

Create a file at app/models/product/notifications.rb with the following:

module Product::Notifications
  extend ActiveSupport::Concern

  included do
    has_many :subscribers, dependent: :destroy
    after_update_commit :notify_subscribers, if: :back_in_stock?
  end

  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

When you include a module in a class, any code inside the included block runs as if it’s part of that class. At the same time, the methods defined in the module become regular methods you can call on objects (instances) of that class.

Now that the code triggering the notification has been extracted into the Notification module, the Product model can be simplified to include the Notifications module.

class Product < ApplicationRecord
  include Notifications

  has_one_attached :featured_image
  has_rich_text :description

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

Concerns are a great way to organize features of your Rails application. As you add more features to the Product, the class will become messy. Instead, we can use Concerns to extract each feature out into a self-contained module like Product::Notifications which contains all the functionality for handling subscribers and how notifications are sent.

Extracting code into concerns also helps make features reusable. For example, we could introduce a new model that also needs subscriber notifications. This module could be used in multiple models to provide the same functionality.