formbuilder_patch.rb

class ActionView::Helpers::FormBuilder

  private
  
    # PATCH: This is a patch of bug in Rails 3 beta 3, and should be removed when we move to Rails 3 final
    def fields_for_with_nested_attributes(association_name, args, block)
      name = "#{object_name}[#{association_name}_attributes]"
      options = args.extract_options!
      association = args.shift
      association = association.to_model if association.respond_to?(:to_model)

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

      if association.respond_to?(:each) # HACK: Replaces if association.is_a?(Array)
        explicit_child_index = options[:child_index]
        output = ActiveSupport::SafeBuffer.new
        association.each do |child|
          output << fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, options, block)
        end
        output
      elsif association
        fields_for_nested_model(name, association, options, block)
      end
    end

end