multiple fields_for behavior

With the following code and result below, why does the cars_attributes
index (i.e. name=owner[cars_attributes][>>>0<<<][color]) increment in
the second fields_for block when only one car is built? Shouldn't it
stay the same because only one car was built?

Is this standard behavior? If so, what is the standard way to force
both indexes to be 0 (or increment together if there are more
records)?

This is an over-simplified example. The reason I want two fields_for
blocks is due the arrangement of partials in another complex form.

Much thanks!

Matt

# owners_controller.rb
# ...
  def new
    @owner = Owner.new
    @owner.cars.build

    respond_to do |format|
      format.html # new.html.erb
      format.xml { render :xml => @owner }
    end
  end
# ...

# owner.rb
class Owner < ActiveRecord::Base
  has_many :cars
  accepts_nested_attributes_for :cars
end

# car.rb
class Car < ActiveRecord::Base
  belongs_to :owner
end

# new.html.erb
<%= form_for(@owner) do |f| %>
  <% if @owner.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@owner.errors.count, "error") %> prohibited
this owner from being saved:</h2>

      <ul>
      <% @owner.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

  <h2>Cars</h2>

  <%= f.fields_for :cars do |car_fields| %>
    <div class="field">
      <%= car_fields.label :color %><br />
      <%= car_fields.text_field :color %>
    </div>
  <% end %>

I solved this using content_for. I put everything in the first
fields_for but the fields I wanted to move to a different partial, I
placed in the content_for block. See the example below.

Hope this helps,

Matt

# example
  <%= f.fields_for :cars do |car_fields| %>
    <div class="field">
      <%= car_fields.label :color %><br />
      <%= car_fields.text_field :color %><br />
    </div>
    <% content_for :plate_fields do %>
      <div class="field">
        <%= car_fields.label :plate %><br />
        <%= car_fields.text_field :plate %><br />
      </div>
    <% end %>
  <% end %>