The thing that changed in 0.8 is that references are now stored as ObjectIds only instead of storing both the table name and the ObjectId (you can still get the old behavior by using dbref=True). However, you would still have to check whether or not the reference was an ObjectId to tell if it was valid.

We're using our fork of https://github.com/elasticsales/flask-mongoengine/ (which I admit is based on a flask-mongoengine version that is a little bit outdated).

Also, will add a link to the comments.