nested_attributes の使用例 （フォーム内で子オブジェクトを参照する方法含む）

application_helper.rb

  def link_to_remove_fields(name, f)
    # See corresponding remove_fields function in application.js
    f.hidden_field(:_destroy) + link_to(name, nil, :onclick => "remove_fields(this)", :remote=>true)
  end

  def link_to_add_fields(name, f, association)
    new_object = f.object.class.reflect_on_association(association).klass.new
    fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
      render(association.to_s.singularize + "_fields", :f => builder)
    end
    html = link_to(raw("<img src='/images/icons/plus.gif' />"), nil, :onclick => h("add_fields(this, \"#{association}\", \"#{escape_javascript(fields)}\")"), :remote => true)
    html << link_to(name, nil, :onclick => h("add_fields(this, \"#{association}\", \"#{escape_javascript(fields)}\")"), :remote => true)
    return html
  end  

detailed_image.rb

class DetailedImage < ActiveRecord::Base
  belongs_to :product
end

detailed_image_fields.html.erb

<%= f.file_field :image, :type=>:file %>
<%= link_to_remove_fields image_tag('/images/icons/cross.gif'), f %>

<% if f.object.id %>
<%= f.object.name %>
<% end %>

f.options[:child_index] returns 1296474703879
But note the above only works for new forms added through link_to_add_fields

The best thing to do is to get the name of the object itself
f.object_name returns product[category_products_attributes][1296474703879]

Or if you really just want the index, gsub it
f.object_name.gsub(/[^0-9]+/,'')

edit.html.erb

<%= form_for([:vendor, @product], :html => {:multipart => true, :class => "std", :name=>'f' }) do |f| %>
  <%= error_messages_for 'product', 'product_vendor' %>

    <!-- ... product fields -->

    <%= f.fields_for :detailed_images do |builder| %>
      <%= render "detailed_image_fields", :f => builder %>
    <% end %>
    <%= link_to_add_fields t('action.add_obj', :obj => DetailedImage.model_name.human), f, :detailed_images %>
<% end %>      

product.rb

class Product < ActiveRecord::Base
  has_many :detailed_images, :dependent => :destroy, :order => "sort_id ASC"
  accepts_nested_attributes_for :detailed_images, :reject_if => lambda { |a| a[:name].blank?}, :allow_destroy => true
end
  

products_controller.rb

@product = Product.find(params[:id)
@product.detailed_images_attributes = [{:something => x}, {:something => y]

# Or, you can do

@product.detailed_images.build({:something => x})
@product.detailed_images.build({:something => y})

# Or

3.times do
 @product.detailed_images.build
end

# Build is different if using a has_one relationship 

@product.build_detailed_image