class ActionView::Helpers::FormBuilder

def initialize(object_name, object, template, options)
  @nested_child_index = {}
  @object_name, @object, @template, @options = object_name, object, template, options
  @default_options = @options ? @options.slice(:index, :namespace, :skip_default_ids, :allow_method_names_outside_object) : {}
  @default_html_options = @default_options.except(:skip_default_ids, :allow_method_names_outside_object)
  convert_to_legacy_options(@options)
  if @object_name.to_s.match(/\[\]$/)
    if (object ||= @template.instance_variable_get("@#{Regexp.last_match.pre_match}")) && object.respond_to?(:to_param)
      @auto_index = object.to_param
    else
      raise ArgumentError, "object[] naming but object param and @object var don't exist or don't respond to to_param: #{object.inspect}"
    end
  end
  @multipart = nil
  @index = options[:index] || options[:child_index]
end