The use case for devise is as follows:

We can add devise and generate the basic authentication by adding the following code to the Gemfile and running the bundle:

gem 'devise', github: 'plataformatec/devise'

We can install devise using the following command line. We can then go ahead and perform the installation of basic configuration files of devise:

:~/curry-nation$rails g devise:install

This will create two files for us: initializers/devise.rb and locales/devise.en.yml. We can now generate our user model:

:~/curry-nation$rails g devise user

The following command line will mount the Devise application routes on the routes.rb file:

config/routes.rb
  devise_for :users

We will now protect selected methods. Devise comes with a set of methods that can be readily used with user-related resources in our application. We will first proceed with the protection of our specific methods inside our recipe model:

app/controllers/recipes_controller.rb
 before_filter :authenticate_user!, only: [:new, :edit, :create, :update, :destroy]

This will allow us to protect the new, edit, create, update, and destroy methods using user authentication. The current_user method allows access to the logged-in user in the session. We can display the e-mail of the user using this method.

Let's write a "create user login" user story as follows:

We can also protect specific methods in views. The if user_signed_in? method is a conditional method provided by Devise. We can use it to check whether the user session is in progress or not. If it is, then we can display the e-mail of the user and the logout link; if not, then display the login and sign-up links:

app/views/layouts/application.html.erb
<ul class="nav navbar-nav pull-right">
    <% if user_signed_in? %>
    <li><%=link_to "#{current_user.email}" %></li>
    <li class="active"><%= link_to "Logout", destroy_user_session_path%></li>
    <%else%>
    <li><%= link_to "Login", new_user_session_path %></li>
    <li class="active"><%= link_to "SignUp", new_user_registration_path%></li>
    <%end%>
</ul>

We can make the methods visible only to the logged-in users. Also, though we have already protected our new and edit methods using authentication, we can hide them altogether from the views, again by using the if user_signed_in? method:

app/views/recipes/index.html.erb
    <% if user_signed_in? %>
    <td><%= link_to 'Edit', edit_recipe_path(recipe), :class=>"btn btn-success btn-small"%></td>
    <td><%= link_to 'Delete', recipe, method: :delete, data: { confirm: 'Are you sure?' }, :class=>"btn btn-danger btn-small" %></td>
        <%end%>