Facebook UserId returned from Azure Mobile Services keeps changing within the same Windows Phone app - facebook

Question

I'm a newbie to app development. I am building a Windows Phone 8.1 app and have followed the tutorial here: http://azure.microsoft.com/en-us/documentation/articles/app-service-mobile-dotnet-backend-windows-store-dotnet-get-started-users-preview/ to add authentication using Facebook. Everything seems to work fine, except that every now and again it appears to stop bringing back any data from my Azure database. Further investigation revealed that the UserId that is being shown from the code below, changes periodically (although I can't quite work out how often it changes).
// Define a member variable for storing the signed-in user.
private MobileServiceUser user;
...
var provider = "Facebook";
...
// Login with the identity provider.
user = await App.MobileService.LoginAsync(provider);
// Create and store the user credentials.
credential = new PasswordCredential(provider,
user.UserId, user.MobileServiceAuthenticationToken);
vault.Add(credential);
...
message = string.Format("You are now logged in - {0}", user.UserId);
var dialog = new MessageDialog(message);
dialog.Commands.Add(new UICommand("OK"));
await dialog.ShowAsync();
This code is identical to the code in the tutorial. The Facebook app settings (on the Facebook developers site) confirm that I am using v2.3 of their API so I should be getting app-scoped UserIds back. I have only ever logged in with one Facebook account, so I would expect the UserId to be the same each time, but they're not. The UserId is prefaced with 'sid:', which someone on the Facebook developers group on Facebook itself says stands for Session ID, which they would expect to change, but if that's the case, I can't work out where to get the actual UserId from that I can then store in my database and do useful things with. I'm sure I must be doing something basic wrong, but I have spent hours Googling this and cannot (unusually) find an answer.
Any help would be greatly appreciated.
Thanks!

Answer 1

So dug deeper. This is how Mobile Apps work (I was thinking from a Mobile Services perspective). The issue here is that the Gateway doesn't provide static SIDs, which is what User.userId provides. The work around to this is listed in the migration doc.
You can only get the Facebook AppId on the server.
ServiceUser user = (ServiceUser) this.User;
FacebookCredentials creds = (await user.GetIdentitiesAsync()).OfType< FacebookCredentials >().FirstOrDefault();
string mobileServicesUserId = creds.Provider + ":" + creds.UserId;
You should note, that this Id is directly connected with your Facebook App registration. If you ever want to migrate your App to a new Facebook App, you'd have to migrate them. You can also use the Facebook AppId to look up the user's global facebook Id via the Facebook Graph API, which you could use between applications. If you don't see yourself using multiple apps, etc., you can use the Facebook AppId just fine.
Hard to tell what's going on to cause you to use a SID instead of the Faceboook token (which like Facebook:10153...).
It may be faster to rip out the code and reimplement the Auth GetStarted. Maybe you missed a step or misconfigured something along the way. If you have the code hosted on github, I can try to take a look.
Another thing you can do is to not trust the user to give you their User id when you save it to a table. On your insert function, you can add it there.
function insert(item, user, request) {
item.id = user.userId;
request.execute();
}
That should, theoretically, be a valid Facebook token. Let me know if that doesn't work; can dig deeper.