app/views/layouts/_admin_banner.html.erb
<div class="admin_banner">
<% if admin? %>
Hi, <%= session[:admin] %>
<%= link_to 'Sign out', destroy_session_path, method: :delete %>
<% else %>
<%= form_tag create_session_path, method: :post do -%>
<%= text_field_tag 'login', nil, class: 'user_field' %>
<%= password_field_tag 'password', nil, class: 'password_field' %>
<%= submit_tag 'Sign in' %>
<% end -%>
<% end %>
</div>