Rails, navigation buttons 'selected' stage

application.html.erb

#...

<ul class="nav">
  <li class="<%= 'active' if current_page?(root_path) %>"><%= link_to "Home", root_path %></li>
  <li class="<%= 'active' if current_page?(about_path) %>"><%= link_to "About", about_path %></li>
  <li class="<%= 'active' if current_page?(contact_path) %>"><%= link_to "Contact", contact_path %></li>
</ul>

#<%= yield %>
#...