According to the documentation (<http://godoc.org/launchpad.net/mgo/v2>) you can obtain the ID of your "Upserted" document if you use the Upsert method.

There is also an Insert method that does not provide this functionality.  

Why is that? What if I want to perform an Insert instead of an Upsert? (or wouldn't ever be any valid reason to want to do that? I'm starting to wonder.)