Ruby on Rails new objects in partial identifier stops at 1 - ruby-on-rails

Question

I have Article(s), that has_many ArticleAssets. Fairly simple. On the edit form for the Article I just want to add new article assets. I don't need to edit the current ones, so I created a partial like this:
<% f.fields_for :article_assets, article_asset do |builder| -%>
<div class="article_asset">
<%= builder.file_field :image %>
<%= builder.check_box :is_flagged, :class => "isFlagged" %> isFlagged
</div>
<% end -%>
No collection, because I only need one object at a time and need no data from the existing article assets. In the form of edit.erb I render the following:
<%= render :partial => 'article_asset', :locals => {:f => f}, :object => ArticleAsset.new %>
This makes one new article asset show up that I can add information to, all cool so far. Important is that this field gets the name-form of article[article_assets_attributes][0][is_flagged]. All good since this will also group the hidden field that always comes with a checkbox in rails to the rest of the fields. Then I have an "Add item" link that does this:
page.insert_html :bottom, :article_assets_fields, :partial => "article_asset", :locals => {:f => f}, :object => ArticleAsset.new
Clicking on this link gives a new field under the created one, as expected, with the name-form of the checkbox field of article[article_assets_attributes][1][is_flagged]. Incremented, that's perfect! Adding another one with the same link however, also gives the same form (also with the identifier of 1, duplicate), which makes submitting the form only have 2 items instead of 3. Does anyone know why this happens and what I can do to solve it?
Ruby on Rails 2.3.11

Answer 1

Nested form 2.3 fail. This one was the bane of my existence for some time, even having watched the railscast etc. Here's my how to:
1) this goes in article.rb
after_update :save_article_assets
def new_article_asset_attributes=(article_asset_attributes)
article_asset_attributes.each do |attributes|
article_assets.build(attributes)
end
end
def existing_article_asset_attributes=(article_asset_attributes)
article_assets.reject(&:new_record?).each do |article_asset|
attributes = article_asset_attributes[article_asset.id.to_s]
if attributes
article_asset.attributes = attributes
else
article_assets.delete(article_asset)
end
end
end
def save_article_assets
article_assets.each do |article_asset|
article_asset.save(false)
end
end
2) In a helper somewhere:
def add_article_asset_link(name)
button_to_function name, :class => "new_green_btn" do |page|
page.insert_html :bottom, :article_assets, :partial => "article_asset", :object => ArticleAsset.new()
end
end
def fields_for_article_asset(article_asset, &block)
prefix = article_asset.new_record? ? 'new' : 'existing'
fields_for("article[#{prefix}_article_asset_attributes][]", article_asset, &block)
end
3) in your partial:
<% fields_for_article_asset(article_asset) do |aa| %>
<tr class="article_asset">
<td><%= aa.text_field :foo %></td>
<td><%= link_to_function "remove", "$(this).up('.article_asset').remove()" %></td>
</tr>
<% end %>
4) in the _form:
<table>
<%= render :partial => "article_asset", :collection => #article.article_assets %>
</table>
<%= add_article_asset_link "Add asset" %>