What does Rails 3 session_store domain :all really do?

Question

Updated question to make it more clear

I understand that you can set the domain of your session_store to share sessions between subdomains like this

Answer 1

OK, the way to accomplish this is to set the domain on the session cookie dynamically. To do this early enough it should be done as rack middleware:

# Custom Domain Cookie
#
# Set the cookie domain to the custom domain if it's present
class CustomDomainCookie
  def initialize(app, default_domain)
    @app = app
    @default_domain = default_domain
  end

  def call(env)
    host = env["HTTP_HOST"].split(':').first
    env["rack.session.options"][:domain] = custom_domain?(host) ? ".#{host}" : "#{@default_domain}"
    @app.call(env)
  end

  def custom_domain?(host)
    host !~ /#{@default_domain.sub(/^\./, '')}/i
  end
end