$(link).previous("input[type=hidden]") is undefined

I am getting this error when trying to use the remove link explained
in the Railscast:
http://railscasts.com/episodes/197-nested-model-form-part-2

error:
$(link).previous("input[type=hidden]") is undefined
remove_fields()applic...1065175 (line 6)
function onclick(event) { remove_fields(this); return false; }()1
(line 2)
[Break on this error] $(link).previous("input[type=hidden]").value =
"1";

in the view file I have :

        <% echantillon_form.fields_for :treatments do |builder| %>
           <%= render "treatment", :f => builder %>
        <% end %>

in the partial I have :

<p class = "treatmentfields">

         <tr>
         <td class ="name"> type d'examen: </td><td></td><td><%=
f.text_field :treatmenttype %></td>
         <td></td>
         </tr>

         <tr>
         <td class ="name"> commentaires sur la preparation : </

<td></td><td><%= f.text_field :preparation %></td>

         <td></td>
         </tr>

         <%= f.hidden_field :_destroy %>

       <tr><td><%= link_to_function "remove", "remove_fields(this)"
%></td></tr>

</p>

and in the appliction.js file I have:

function remove_fields(link) {

  $(link).previous("input[type=hidden]").value = "1";
  $(link).up(".treatmentfields").hide();

}

Looks like because the hidden field and the link don't have the same
parent - the previous function finds you the previous sibling element
(matching the selector if specified)

Fred

I had to restructure my page in html so that the link did follow the
hidden input field. It works now but when I follow the further
instructions in the railscast to have an "add" link to add an object,
I get this error:

undefined method `render_partial' for #<ActionView::ReloadableTemplate:
0x9f80d28>

this is my view:

        <% echantillon_form.fields_for :treatments do |builder| %>
           <%= render "treatment", :f => builder %>
        <% end %>

      <%= link_to_add_fields "Add treatment",
echantillon_form, :treatments %>

the problem was that I didn't have "_fields" as part of the name of my
partial.