gistfile1.builder

# views/resources/new.html.haml
= form_for @resource do |form|
  = form.fields_for :fields do |field_form|
    render :partial => "field_template", :locals => { :form => field_form }
  = link_to "Add new resource field", empty_field_resources_path, :remote => true

# controllers/resources_controller.rb
class ResourcesController < ApplicationController
    def empty_field
      respond_with do |format|
        format.js
      end
    end
end

# empty_field.js.erb
<% 
    template = nil
    form_for(Resource.new) do |f|
        f.fields_for :fields, ResourceField.new, :child_index => Time.now.to_i do |form|
            template = render :partial => "field_template", :locals => { :form => form }
        end
    end
%>
$("#resource_fields").append("<%= raw escape_javascript( template )%>");