rails source fields_for has an options hash not defined anywhere

Hey all,

I looked at source code for fields_for:

def fields_for(record_name, record_object = nil, fields_options = {},
&block)
  fields_options, record_object = record_object, nil if
record_object.is_a?(Hash) && record_object.extractable_options?
  fields_options[:builder] ||= options[:builder]
  fields_options[:parent_builder] = self
  fields_options[:namespace] =
fields_options[:parent_builder].options[:namespace]

  case record_name
  when String, Symbol
    if nested_attributes_association?(record_name)
      return fields_for_with_nested_attributes(record_name,
record_object, fields_options, block)
    end
  else
    record_object = record_name.is_a?(Array) ? record_name.last :
record_name
    record_name = ActiveModel::Naming.param_key(record_object)
  end

  index = if options.has_key?(:index)
    "[#{options[:index]}]"
  elsif defined?(@auto_index)
    self.object_name = @object_name.to_s.sub(%r\[\]$/,"")
    "[#{@auto_index}]"
  end
  record_name = "#{object_name}#{index}[#{record_name}]"

  @template.fields_for(record_name, record_object, fields_options,
&block)
end

Notice this part:

options[:builder]

this options hash is no where to be defined. In argument list, there's
only a fields_options hash. Reason why this matters to me is because I
am trying to override this method in my application_helper.rb in order
to extend it.

I want to know this too. Did you find out?

I want to know this too. Did you find out?

That fields_for method is an instance method of FormBuilder, which has an attr_accessor :options

Fred