Rails accepts_nested_attributes_for with f.fields_for and AJAX

I'm curious how to properly use accepts_nested_attributes_for and f.fields_for.

views/orders/new.html.erb

<%= form_for @order, html:{role: "form"} do |f| %>

  <%= f.submit "Don't push...", remote: true %>
  <%= f.text_field :invoice %>
  <%= f.text_field :ordered_on %>
  <%= f.text_field :delivered_on %>

  <table  id='order_form'>
    <h3>Details</h3>
    <tbody>
      <%= render 'order_details/details', f: f %>
    </tbody>
    <%= link_to 'add line', new_order_detail_path(company_id: params[:company_id]), remote: true %>
    <%= link_to 'new box', new_box_path, remote: true %>
  </table>

<% end %>

views/order_details/_details.html.erb

<tr class='row0'>
  <%= f.fields_for :order_details, child_index: child_index do |d| %>
    <td><%= d.collection_select :box_id, @boxes, :id, :uid, {},
                                { name: "box_id", class: 'form-control'} %></td>
    <td><%= d.text_field :quantity, class: 'form-control' %></td>
    <td><%= d.text_field :box_price, class: 'form-control' %></td>
    <td><%= d.text_field :cb_price, class: 'form-control' %></td>
    <td><%= d.text_field :mould_fees, class: 'form-control' %></td>
    <td>$$$</td>
  <% end %>
</tr>
<tr class='box0'>
  <td colspan="6">&#8594; <b><%= @b.uid %></b> | length: <%= @b.length %> | width: <%= @b.width %> | height: <%= @b.height %> | weight: <%= @b.weight %></td>
</tr>

controllers/orders_controller.rb (I'm pretty sure this is wrong... any help here would be greatly appreciated)

def create
  @order = Order.create(params[:order])
  if @order.save
    flash[:success] = "Order #{@order.invoice} added!"
    redirect_to current_user      
  else
    render 'orders/new'
  end
end

models/order.rb

class Order < ActiveRecord::Base
  attr_accessible ..., :order_details_attributes
  has_many :order_details
  accepts_nested_attributes_for :order_details
end

The only way I've been able to get the partial to play nice is if I actually call the fields_for as fields_for Order.new.order_details.build. But that doesn't build the nested object at all. I need to use the f.fields_for nomenclature to build the Order, and the OrderDetail. I can only build one, though. Which is my next issue.

See how there are buttons in there? It AJAXs rows into the form. If you click add line, I get

NameError in Order_details#new
Showing D:/Dropbox/Apps/rails_projects/erbv2/app/views/order_details/new.js.erb where line #3 raised:
undefined local variable or method `f' for #<#<Class:0x5cf0a18>:0x5cbd718>

views/orders/add_detail.js.erb

$('#order_form tr.total').before("<%= j render partial: 'orders/details', locals: {f: @f, child_index: @ci} %>")

I don't know how to define f... I checked out Rails AJAX: My partial needs a FormBuilder instance and a few others.

Any suggestions on how I should handle this? Using the code I have here... I was able to create a new order, with an associated order_details, but the box_id didn't save, and the company_id didn't save. I know this is kind of nasty, but I don't know where else to go.

UPDATE

routes:

resources :orders do
  collection { get :add_detail }
end

this is way better than having a separate resource for the details.  I didn't think of this before!

HTML form:

<%= form_for @order, company_id: params[:company_id], html:{role: "form"} do |f| %>
  f. ...
  <%= render partial: 'details', locals: { f: f } %> #first child
  <%= link_to 'add line', add_detail_orders_path(company_id: params[:company_id]), remote: true %> #add subsequent children
<% end %>

Orders Controller:

def add_detail
  @order = Order.build
  @boxes = Company.find(params[:company_id]).boxes
  @b = @boxes.first
  @ci = Time.now.to_i
  respond_with(@order, @boxes, @b, @ci)
end

_details partial

<%= form_for @order do |f| %>
  <%= f.fields_for :order_details, child_index: @ci do |d| %>
    <td><%= d.collection_select :box_id, @boxes, :id, :uid, {},
                                {class: 'form-control'} %></td>
    <td><%= d.text_field :quantity, class: 'form-control' %></td>
    <td><%= d.text_field :box_price, class: 'form-control' %></td>
    <td><%= d.text_field :cb_price, class: 'form-control' %></td>
    <td><%= d.text_field :mould_fees, class: 'form-control' %></td>
    <td>$$$</td>
  <% end %>
<% end %>