Resources

Notifications Gem - https://github.com/rails-engine/notifications
Source - https://github.com/driftingruby/136-user-notifications

Summary

Terminalrails g notifications:install
rails db:migrate

rails g notifications:controllers
rails g notifications:views

Gemfilegem 'notifications'

layouts/application.html.erb<div class='container'>
  <ul class='navigation'>
  <% if user_signed_in? %>
    <li><%= link_to pluralize(Notification.unread_count(current_user), 'Notification'), notifications_path %></li>
    <li><%= link_to current_user.email, main_app.edit_user_registration_path %></li>
    <li><%= link_to 'Logout', main_app.destroy_user_session_path, method: :delete %></li>
  <% else %>
    <li><%= link_to 'Login', main_app.new_user_session_path %></li>
  <% end %>
  </ul>
  <%= yield %>
</div>

models/comment.rbclass Comment < ApplicationRecord
  belongs_to :post
  belongs_to :user

  after_commit :create_notifications, on: :create

  private

  def create_notifications
    Notification.create do |notification|
      notification.notify_type = 'post'
      notification.actor = self.user
      notification.user = self.post.user
      notification.target = self
      notification.second_target = self.post
    end
  end
end

views/notifications/_post.html.erb<div class=''>
  <%= link_to notification.actor.email, main_app.user_path(notification.actor) %> has commented in
  <%= link_to notification.second_target.content, main_app.user_post_path(notification.user, notification.second_target) %>
</div>

<div class=''>
  <%= notification.target.content %>
</div>

db/routes.rb  mount Notifications::Engine => "/notifications"