gistfile1.rb

class IncrementJumpingFormBuilder < ActionView::Helpers::FormBuilder  

  def fields_for_with_nested_attributes(association_name, args, block)
    name = "#{object_name}[#{association_name}_attributes]"
    association = args.first

    if association.respond_to?(:new_record?)
      association = [association] if @object.send(association_name).is_a?(Array)
    elsif !association.is_a?(Array)
      association = @object.send(association_name)
    end

    if association.is_a?(Array)
      explicit_child_index     = args.last[:child_index] if args.last.is_a?(Hash)
      nested_child_index_start = args.last[:child_index_start] if args.last.is_a(Hash)
      nested_child_index_start ||= -1

      association.map do |child|
        fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name, nested_child_index_start)}]", child, args, block)
      end.join
    elsif association
      fields_for_nested_model(name, association, args, block)
    end
  end

  def nested_child_index(name, nested_child_index_start)    
    Rails.logger.debug("Nested Child Index: #{name}, #{nested_child_index_start}")
    
    @nested_child_index[name] ||= nested_child_index_start
    @nested_child_index[name] += 1
  end

end