christineyen/form_helper.rb

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

  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
  ...

private
  def nested_attributes_association?(association_name)
    @object.respond_to?("#{association_name}_attributes=")
  end

# rails/actionpack/lib/action_view/helpers/form_helper.rb

## redirecting.rb
def redirect_to(options = {}, response_status = {}) #:doc:
  raise ActionControllerError.new("Cannot redirect to nil!") if options.nil?
  raise AbstractController::DoubleRenderError if response_body

  self.status        = _extract_redirect_to_status(options, response_status)
  self.location      = _compute_redirect_to_location(options)
  self.response_body = "<html><body>You are being <a href=\"#{ERB::Util.h(location)}\">redirected</a>.</body></html>"
end

# rails/actionpack/lib/action_controller/metal/redirecting.rb
	def fields_for(record_name, record_object = nil, fields_options = {}, &block)
	fields_options, record_object = record_object, nil if record_object.is_a?(Hash)
	fields_options[:builder] \|\|= options[:builder]
	fields_options[:parent_builder] = self

	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
	...

	private
	def nested_attributes_association?(association_name)
	@object.respond_to?("#{association_name}_attributes=")
	end

	# rails/actionpack/lib/action_view/helpers/form_helper.rb
	def redirect_to(options = {}, response_status = {}) #:doc:
	raise ActionControllerError.new("Cannot redirect to nil!") if options.nil?
	raise AbstractController::DoubleRenderError if response_body

	self.status = _extract_redirect_to_status(options, response_status)
	self.location = _compute_redirect_to_location(options)
	self.response_body = "<html><body>You are being <a href=\"#{ERB::Util.h(location)}\">redirected</a>.</body></html>"
	end

	# rails/actionpack/lib/action_controller/metal/redirecting.rb