I have the same issue. But to clearify. Are you saying I have to either add a new field to my docs in my collection that is the string of my users ObjectId ?
You wrote that its possible to use $search to get around this. Please explain how to do that exactly.
Or is there any other way in solving this. I mean, this has to be something that pretty much everyone will bump into. Having something like a user field with a ObjectID refering the a users collection.
It would be a bit frustrating if I would have to duplicate the user field and have userString field.