class Padrino::Helpers::FormBuilder::AbstractFormBuilder

def fields_for(child_association, collection=nil, options={}, &block)
  default_collection = self.object.send(child_association)
  collection ||= default_collection
  include_index = default_collection.respond_to?(:each)
  nested_options = { :parent => self, :association => child_association }
  Array(collection).each_with_index.inject(ActiveSupport::SafeBuffer.new) do |all,(child_instance,index)|
    nested_options[:index] = options[:index] || (include_index ? index : nil)
    all << @template.fields_for(child_instance,  { :nested => nested_options, :builder => self.class }, &block) << "\n"
  end
end