Challenge 2: Broadcasting on Create — Solution

# app/models/notification.rb
class Notification < ApplicationRecord
  belongs_to :user
  after_create_commit :broadcast_notification

  private

  def broadcast_notification
    NotificationsChannel.broadcast_to(user, NotificationSerializer.new(self).as_json)
  end
end

=begin
Notes:
- after_create_commit (not after_create) waits until the surrounding
  database transaction actually commits before broadcasting -- matching
  the chapter's warning: broadcasting on after_create risks telling a
  client about a record a later rollback then undoes.
- broadcast_to(user, ...) targets exactly the stream Challenge 1's
  NotificationsChannel opened via stream_for current_user -- the same user
  object is used on both the subscribing and broadcasting side, which is
  what makes them the same stream.
- NotificationSerializer.new(self).as_json reuses a serializer the same
  way rails2-5's chapter reused CommentSerializer for both the JSON API
  and the broadcast payload -- one definition of "what a notification looks
  like," regardless of which channel (HTTP or WebSocket) delivers it.
=end