Nested form for a serialized has_many relationship

00000_create_users.rb

class AddBillItemsToUsers < ActiveRecord::Migration
  def self.up
    add_column :users, :bill_items, :string
  end

  def self.down
    remove_column :users, :bill_items
  end
end

bad_output.html

<form action="/users/1" method="post">
<input id="user_bill_items_line_item" name="user[bill_items][line_item]" type="text" />
<input id="user_bill_items_subtotal" name="user[bill_items][subtotal]" type="text" />
</form> 

bill_item.rb

class BillItem
  attr_accessor :line_item, :subtotal
end

form.html.haml

= form_for(@user) do |f|
  = f.fields_for :bill_items do |bill_f|
    = f.text_field :line_item
    = f.text_field :subtotal
  ...

good_output.html

<form action="/users/1" method="post">
<input id="user_bill_items_attributes_0_line_item" name="user[bill_items_attributes][0][line_item]" type="text" value="Walgreens - Garden Hose" />
<input id="user_bill_items_attributes_0_subtotal" name="user[bill_items_attributes][0][subtotal]" type="text" value="11.0" />
</form> 

user-v2.rb

class User < ActiveRecord::Base
  serialize :bill_items, Array
  ...
  def bill_items_attributes=(attributes)
    # do something
  end
end

user.rb

class User < ActiveRecord::Base
  serialize :bill_items, Array
  ...
end