auto_complete REST 'show' action

Upon selecting a match and hitting enter, how do I invoke the 'show'
action to view the individual resource? This is what I have so far:

<% form_tag :action => :show do %>
    <%= text_field_with_auto_complete :user, :last_name %>
    <%= submit_tag 'Go' %>
  <% end %>

I get the following error: Couldn't find User without an ID

Any suggestions?