http://guestbook.co.za/ is also available at http://www.guestbook.co.za/. We wanted our users to remain logged in if they’re using GuestBook without the ‘www.’ subdomain and then enter it or visit the site from a link.

To do so we needed to share the session between www.guestbook.co.za and guestbook.co.za. This sounds really difficult but thankfully all it takes is prepending the domain name of your cookie with a period (.)

Here’s how to do it in Rails:

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# In your production.rb file
config.action_controller.session = {
  :session_domain => ".your-domain.tld",
  :session_key    => "_yourapp",
  :secret         => "somehugesecretkeythathastobemorethanthirtycharacterslong"
}

Thanks for the original post I got this from!