using fields_for in several places

I have a simple model

class Ad < ActiveRecord::Base
   has_many :ad_items
end

class AdItem < ActiveRecord::Base
   belongs_to :ad
end

I have an "ads/new" view, that shows me the form for creating the new ad and adding some items to it

The .html.erb code is like a following:

<% form_for @ad, do |ad_form| %>
   <!-- some html -->

   <% ad_form.fields_for :ad_items do |f| %>
      <%= f.text_area "com开发者_如何学JAVAment", :class => "comment", :rows => "5" %>
   <% end %>

   <!-- some other html -->

   <% ad_form.fields_for :ad_items do |f| %>
      <% render :partial => "detailed_item_settings", :locals => {:f => f} %>
   <% end %>
<% end %>

When the ad has one item ...

def new
   @ad = session[:user].ads.build

   # Create one item for the ad. Another items will be created on the
   # client side
   @ad.ad_items.build

   # standard stuff ...
end

... resulting HTML, will look like this:

<form ... >
   <!-- some html -->

   <textarea id="ad_items_attributes_0_comment" name="ad[ad_items_attributes][0][comment]" />

   <!-- some other html -->

   <!-- "detailed_item_settings" partial's content -->
      <textarea id="ad_ad_items_attributes_1_desc" name="ad[ad_items_attributes][1][desc]" />
   <!-- end -->
</form>

As it states in the code, i use fields_for method twice, because of the HTML structure, that i must follow

For the second "fields_for" call, index for "item" is already 1, not 0, as i expect.

It's like, that by calling "fields_for" method, some internal counter will be incremented ...

But this is a little strange behaviour ...

I've tried to set :index => 0 for fields_for, but all stays the same ...

What's wrong here ?

---

You can set index manually for each item, but you have to iterate through your items for that to get item index:

  <% ad_form.fields_for :ad_items do |f| %>
     <%= f.text_area "comment", :class => "comment", :rows => "5" %>
  <% end %>
  ...
  <% ad_items.each_with_index do |item, i| %>
    <% ad_form.fields_for :ad_items, item, :child_index => i do |f| %>
      <% render :partial => "detailed_item_settings", :locals => {:f => f} %>
    <% end %>
  <% end %>