adding errors to an autocomplete field and ensuring it is wrapped with fieldWithErrors

So I'm failing to find how to do this the 'rails-way'
Here's the problem, it's easier to show it in an example...but
basically I'm having issues with validation errors highlighting
(wrapping) with the 'fieldWithErrors' div and multiple models in a
belongs_to relationship.
So here's the example:
models:
   child.rb
      belongs_to :parent
      validates_presence_of :parent
      validates_presence_of :name

   parent.rb
      has_many :children

controller:
  # basic scaffold style controller with an method to support ajax:
  indexed_auto_complete_for :child, :parent, :name

view:
  <label for="parent_name">parent</label><br/>
  <%= text_field_with_auto_complete :parent, :name,
                                    { :value => @parent_name },
                                    { :skip_style => true,
                                      :after_update_element =>
'auto_complete_on_select' } %>
  <% # hidden field is updated via ajax %>
  <%= hidden_field :child, :parent_id %>

modified lib/auto_complete.rb
    # http://ricardo.pacheco.name/blog/articles/2006/09/17/using-autocomplete-with-belongs_to-fields-part-ii
    def indexed_auto_complete_for(parent, object, method, options =
{})
      define_method("auto_complete_for_#{object}_#{method}") do
        find_options = {
          :conditions => [ "LOWER(#{method}) LIKE ?", '%' +
params[object][method].downcase + '%' ],
          :order => "#{method} ASC",
          :limit => 10 }.merge!(options)

        @items = object.to_s.camelize.constantize.find(:all,
find_options)

        render :inline => "<%= indexed_auto_complete_result @items,
'#{parent}_#{object}_id', '#{method}', 'id' %>"
      end
    end

modified application.js
// this methods are to allow autocomplete on :belongs_to a bit easier,
requires:
// - application.js - auto_complete_on_select
// - auto_complete_macros.rb - indexed_auto_complete_result
// - auto_complete.rb - indexed_auto_complete_for
function auto_complete_on_select(element, selectedElement)
{
  var entityParts = selectedElement.id.split('::');
  var entityType = entityParts[0];
  var entityId = entityParts[1];
  document.getElementById(entityType).value = entityId;
}

So all of this renders my form as:
  <label for="child_parent_id">parent</label><br/>

  <input id="parent_name" name="parent[name]" size="30" type="text" /

<div class="auto_complete" id="parent_name_auto_complete"></
<script type="text/javascript">

//<![CDATA[
var parent_name_auto_completer = new Ajax.Autocompleter('parent_name',
'parent_name_auto_complete', '/children/
auto_complete_for_parent_name',
{afterUpdateElement:auto_complete_on_select,
parameters:'authenticity_token=' +
encodeURIComponent('2789eb52554dda1a74e3363f76966983ab806fce')})
//]]>
</script>
    <input id="child_parent_id" name="child[parent_id]" type="hidden" /

</p>

What's the problem?
Well when I submit the form, the validation runs, and I get an
error...but the fieldWithErrors tag doesn't get wrapped on the
parent_name field.

I'm sure I'm doing something 'wrong' but I'm not clear on what. I've
got a feeling that having the field without a model directly attached
is not rails approved, but I'm confused about that since the
collection_select is such a common thing...this would seem to be just
a direct extension to that.

I've tried adding to the errors hash in the model (this doesn't work):
    def after_validation
      parent_error = errors.on(:parent)
      if parent_error
        errors.add(:parent_name, parent_error)
        errors.add('parent_name', parent_error)
        errors.add('parent[name]', parent_error)
      end
    end

Now I'm certain I can hack my way out with either:
- add some sort of accessor or something like that to child.rb
- re-do my modifications to the autocomplete stuff to end up with an
id like child[parent_name]
- change my view to this..but this is definitely an ugly non-
extensible hack:
  <% if @child.errors.on(:parent) -%><div class='fieldWithErrors'><%
end %>
  <%= text_field_with_auto_complete :parent, :name,
                                    { :value => @parent_name },
                                    { :skip_style => true,
                                      :after_update_element =>
'auto_complete_on_select' } %>
  <% if @child.errors.on(:parent) -%></div><% end %>

Thanks in advance,
Dale

So this is the rails way...go watch: http://railscasts.com/episodes/102
Thanks Ryan.