devise

_admin_menu.html.erb

<!-- 
Used for https://github.com/rubykurs/bootstrap
branch auth

@Gemfile
gem 'devise'
$ rails g devise:install
$ rails g devise admin
$ rake db:migrate
-->

<ul class="nav secondary-nav">
  <% if admin_signed_in?  %>
    <li><%= link_to "Sign out", destroy_admin_session_path, :method => :delete %> </li>
  <% else  %>
    <li><%= link_to "Admin", new_admin_session_path %></li>
  <% end  %>
</ul>

application.html.erb

<!--
or common/_header.html.erb if body header is extracted there.
-->

  <div class="topbar">
    <div class="fill">
      <div class="container">
         ...
        <%= render :partial => 'common/admin_menu' %>
      </div>
    </div>
  </div>