How to use oauth from a facebook app

I'm developing a facebook app with rails that uses external api开发者_高级运维s from my own domain. The problem is that my domain requires authentication, which is done via oauth. It's not clear to me how to deal with this pattern. I'm not sure I can make oauth calls from a facebook app, thus requiring two separate registrations. Is there a way to pass a facebook access token so that I know the user is authenticated through facebook?

---

If you are using (or can use) Rails 3.0+, devise has a good section on how to authenticate via facebook or a google account.

Once a user has used this method to authenticate to your webapp, their session is handled in the same way a regular login session is, so you can just use current_user.nil? or user_signed_in? helpers to determine if the users are authenticated or not.