app/controllers/application_controller.rb

@@ -1,2 +1,9 @@
 class ApplicationController < ActionController::Base
+  before_action :authenticate_user
+
+  private
+
+  def authenticate_user
+    Current.user = User.find_by(id: session[:current_user_id])
+  end
 end

app/models/current.rb

@@ -0,0 +1,3 @@
+class Current < ActiveSupport::CurrentAttributes
+  attribute :user
+end

app/views/layouts/application.html.erb

@@ -19,7 +19,11 @@
         <li class="mr-6"><%= link_to "Credit Card Bills", credit_card_bills_path, class: "text-white" %></li>
         <li class="mr-6"><%= link_to "Incomes", incomes_path, class: "text-white" %></li>
         <li class="mr-6"><%= link_to "Members", members_path, class: "text-white" %></li>
-        <li class="mr-6"><%= link_to "Log out", session_path, data: {turbo_method: :delete}, class: "text-white" %></li>
+        <% if Current.user %>
+          <li class="mr-6"><%= link_to "Log out", session_path, data: {turbo_method: :delete}, class: "text-white" %></li>
+        <% else %>
+          <li class="mr-6"><%= link_to "Log in", new_session_path, class: "text-white" %></li>
+        <% end %>
       </ul>
     </nav>