gistfile1.rb

FORM

= form_for @trip do |f|
  %p
    = add_paddler_kayak_link("Add a paddler", f)
  .actions
    = f.submit 'Save'

PADDLER_KAYAK PARTIAL

%p.fields
  = select_tag "paddler_kayak[paddler][]", options_from_collection_for_select(@paddlers, "id", "name"), :prompt => "Select Paddler"
  = select_tag "paddler_kayak[kayak][]", options_from_collection_for_select(@kayaks, "id", "full_name"), :prompt => "Select Kayak"
  = link_to_function "remove", "remove_fields(this)"

TRIP MODEL

class Trip < ActiveRecord::Base
  has_many :paddlers
  serialize :paddler_kayak, Hash

  def self.serialized_attr_accessor(*args)
    args.each do |method_name|
      eval "
        def #{method_name}
          (self.paddler_kayak || {})[:#{method_name}]
        end
        def #{method_name}=(value)
          self.paddler_kayak ||= {}
          self.paddler_kayak[:#{method_name}] = value
        end
      "
    end
  end

  serialized_attr_accessor :paddler, :kayak
end

TRIPS CONTROLLER

def create
    @trip = Trip.new(params[:trip])
    @trip.paddler_kayak = params[:paddler_kayak]

    respond_to do |format|
      if @trip.save
        format.html { redirect_to(@trip, :notice => 'Trip was successfully created.') }
        format.xml  { render :xml => @trip, :status => :created, :location => @trip }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @trip.errors, :status => :unprocessable_entity }
      end
    end
  end

TRIP HELPER

module TripsHelper
  def add_paddler_kayak_link(name, f)
    new_object = Paddler.new
    fields = f.fields_for(:paddler_kayak, new_object, :child_index => "new_paddler") do |builder|
      render("paddler_kayak")#, :locals => {:f => builder}
    end
    link_to_function(name, "add_fields(this, 'Paddler', '#{escape_javascript(fields)}')")
  end
end 

JS

function add_fields(link, association, content) {
    var new_id = new Date().getTime();
    var regexp = new RegExp("new_" + association, "g");
    $(link).parent().children(":first").after(content.replace(regexp, new_id));
}