Object in hidden_field_tag becomes nil when partial is called into an index (yet, works in individual Show view)
<% content_for(:scripts) do %>
  <%= javascript_include_tag 'rating_ballot'%>
<% end %>

<div id="talesFeedHome">
  <p class="notice"><%= notice %></p>
  <%= render @tales.sort_by { |tale| tale.created_at }.reverse %>
</div>

<p class="clear">&nbsp;</p>

<% if signed_in? %>

      <div id="homeTales">
        <ul>
          <div id="taleShow">
            <div id="controlPanel">
              <li id="taleUserName"><%= tale.user.name %></li>
              <li id="averageRating"> Your Rating:<br /><%= render "tales/stars" %></li>
            </div>
      <div id="taleDisplay">
        <li><%= link_to(tale) do %>
          <span><%= tale.title %> </span>
          <span><%= tale.content %></span>
        <% end %>
        </li> <br />
      </div>
    </div>
  </ul>
</div>

<% else %>
  ...

<div id="starRating">
  <%= form_for(rating_ballot, :remote => true, :html => { :class => 'rating_ballot' }) do |f| %>
    <%= f.label("value_1", content_tag(:span, '1'), {:class=>"rating", :id=>"1"}) %>
      <%= radio_button_tag("rating[value]", 1, current_user_rating == 1, :class => 'rating_button') %>
    <%= f.label("value_2", content_tag(:span, '2'), {:class=>"rating", :id=>"2"}) %>
      <%= radio_button_tag("rating[value]", 2, current_user_rating == 2, :class => 'rating_button') %>
    <%= f.label("value_3", content_tag(:span, '3'), {:class=>"rating", :id=>"3"}) %>
      <%= radio_button_tag("rating[value]", 3, current_user_rating == 3, :class => 'rating_button') %>
    <%= f.label("value_4", content_tag(:span, '4'), {:class=>"rating", :id=>"4"}) %>
      <%= radio_button_tag("rating[value]", 4, current_user_rating == 4, :class => 'rating_button') %>
    <%= f.label("value_5", content_tag(:span, '5'), {:class=>"rating", :id=>"5"}) %>
      <%= radio_button_tag("rating[value]", 5, current_user_rating == 5, :class => 'rating_button') %>

    <%= hidden_field_tag(:tale_id, @tale.id) %>
  <% end %>
</div>

Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id

class PagesController < ApplicationController
  def home
    @tales = Tale.all
  end
end

class TalesController < ApplicationController
  respond_to :html, :js

  def new
    @tale = Tale.new
  end

  def show
    @tale = Tale.find(params[:id])
  end

  def index
    @tales = Tale.all
    @tale = Tale.find(params[:id])
  end
...

class RatingsController < ApplicationController
   before_filter :authenticate_user!
   respond_to :html, :js

   def create
     @tale = Tale.find_by_id(params[:tale_id])
     @rating = Rating.new(params[:rating])
     @rating.tale_id = @tale.id
     @rating.user_id = current_user.id

     if @rating.save
         respond_to do |format|
             format.html { redirect_to @tale, :notice => "Your rating has been saved" }
             format.js
         end
     end
   end

   def update
     @rating = current_user.ratings.find_by_tale_id(params[:tale_id])
     @tale = @rating.tale


     if @tale and @rating.update_attributes(params[:rating])
        respond_to do |format|
            format.html { redirect_to @tale, :notice => "Your rating has been updated" }
            format.js
        end
     end
   end
end