I’ve been working on implementing single sign on in Lessonly (a Rails 4.2 app), specifically with SAML (because that’s what the clients need). Omniauth has been making it fairly easy to do, which has been great.

However, I had need of keeping track of a return_url. This can get weird because with SSO, the service provider (SP) sends the user to a different place on the web, the identity provider (IdP), which then sends the user back to your SP. Keeping track of a return_url is not reliable through these requests.

There is one thing that I tend not to think about when developing Rails apps - the session. In general, I believe that using the session is a bad idea. The big exception being that I use it to keep track of the current logged in user.

Since I don’t generally use the session, this option escaped my mind for a while when I was trying to figure out this particular problem. However, it works very well here.

Omniauth allows us to define a setup action which runs before the authentication request is sent:

# config/initializers/omniauth.rb
provider :saml, setup: true

# config/routes.rb
get "/auth/:provider/setup" => "sessions#sso_setup"

# controllers/sessions_controller.rb
def sso_setup
  session[:sso_return_url] = params[:return_url] if params[:return_url]
  # additional setup...

We can then use the return_url in the session upon authentication:

# config/routes.rb
post "/auth/:provider/callback" => "sessions#sso", as: :sso_callback

# controllers/sessions_controller.rb
def sso
  # authenticate...
  redirect_to sessions[:return_url]

Just as simple as that.