Once you've installed Ruby, Ruby on Rails, and RubyGems, download a copy of the CS50 Library from RubyGems via:

    $ gem install cs50

Now, create a new project, like <tt>cs50id</tt>:

    $ rails new cs50id

Now, add the CS50 library to the project by adding the following to <tt>/Gemfile</tt>:

    gem 'cs50'

The project has been configured to use the CS50 Library, so a new controller, called <tt>Auth</tt>, can be created to handle
user authentication:

    $ rails generate controller Auth

In order to log in, users will be redirected to CS50 ID, via a link generated by the CS50 Library. We must specify the
location of a temporary directory to store login information, the registered TRUST_ROOT and RETURN_TO, and the object
representing the current session. In <tt>/add/controllers/auth_controller.rb</tt>, create a new action for logging in:

    def login
        # user already logged in, redirect to index
        if (session[:id])
            redirect_to :action => :index

        # redirect to CS50 ID
        else
            redirect_to CS50.getLoginUrl(Rails.root.join("tmp"), "http://localhost:3000", 
                                        "http://localhost:3000/auth/return", session)
        end
    end

Once the user authenticates via CS50 ID, they'll be brought back to `http://localhost:3000/auth/return`. At this point,
we can retrieve information associated with the user that successfully logged in. To do so, we must again specify our
temporary directory and RETURN_TO, as well as the current session and params objects (where the user information is
stored). Because our RETURN_TO is <tt>/auth/return</tt>, we need to create a <tt>return</tt> action:

    def return
        # get authenticated user information
        user = CS50.getUser(Rails.root.join("tmp"), "http://localhost:3000/auth/return", 
                            session, params)

        # remember which user, if any, logged in
        if (user)
            session[:id] = user[:id]

            if (user[:email])
                session[:email] = user[:email]
            end

            if (user[:name])
                session[:name] = user[:name]
            end
        end

        redirect_to :action => :index
    end

Because we stored user information in the session, a logout is accomplished by clearing the session.

    def logout
        # clear the user's information from the session
        session[:id] = nil
        session[:email] = nil
        session[:name] = nil

        redirect_to :action => :index
    end

We have specified an <tt>index</tt> action in both <tt>login</tt> and <tt>logout</tt>, so we must create one:

    def index
    end

Now, create the corresponding view, <tt>/app/views/auth/index.html.erb</tt>:

    <% if (session[:id]) %>
        You are logged in. <%= link_to "Log out", :action => "logout" %><br />
        Your unique ID for this site is <strong><%= session[:id] %></strong><br />
        Your email address is <strong><%= session[:email] %></strong><br />
    <% else %>
        You are not logged in. <%= link_to "Log in", :action => "login" %>
    <% end %>

Finally, we must set up routes to the four actions we've created, so add the following to <tt>/app/config/routes.rb</tt>

    match 'auth' => 'auth#index'
    match 'auth/login' => 'auth#login'
    match 'auth/logout' => 'auth#logout'
    match 'auth/return' => 'auth#return'

To run the app, start the Rails server with:

    $ rails s

Now, navigate to </tt>http://localhost:3000/auth</tt>. You should see a link to log in, at which point you'll be redirected to
CS50 ID. After authenticating, you'll be brought back to your app, and you'll see your user information and a logout link!