The WriteResult from an upsert displays in the shell like so:

res = db.c.update(..., ..., {upsert: true})
printjson(res)
{
	"nMatched" : 0,
	"nUpserted" : 1,
	"nModified" : 0,
	"_id" : ObjectId("5ca4b9271da31f1c4f6cc755")
}

However attempting to access the _id returns undefined:

printjson(res._id)
undefined

The returned _id can be accessed as res.getUpsertedId()._id, but the value printed for res and the documentation indicate that it should be available as res._id.