======== hoge.html ========
<%= render :partial => 'login' %>


======== _login.html ========
<% if user? %>
  <%= _("welcome %{firstname} %{lastname}: ") % {:firstname => current_user.firstname, :lastname => current_user.lastname} %>
  <%= link_to _('logout'), :controller => 'user', :action => 'logout' %> |
  <%= link_to _('Change password'), :controller => 'user', :action => 'change_password' %>
<% else %>
  <%= store_location %>
  <%= link_to _('login'), :controller => 'user', :action => 'login' %>
<% end %>