back_or_home_path.rb

def back_or_home_path
  referrer = controller.respond_to?(:request) && controller.request.env['HTTP_REFERER']
  local_referrer = referrer && URI.parse(referrer).host == controller.request.host
  if referrer && local_referrer
    referrer
  else
    root_path
  end
end

def back_or_home_path
  controller.respond_to?(:request) && referer_is_local ? referer : root_path
end

private
  def referer_is_local
    URI.parse(referer).host == controller.request.host
  end

  def referer
    @referer ||= controller.request.env['HTTP_REFERER']
  end

There is a request.referrer method

http://stackoverflow.com/a/8136648

URI(request.referer).host == request.host