When you persist self-referential structures in the database with auto-incommitted primary keys, you've got to get one row, get the primary key back. Everything's got to be done in the very specific order. And it took a very long time to get it right. It has a couple of little chinks in the armor, a couple of cases that you might have to drop it to call it flush explicitly. But we never get bugs with the unit of work stuff. If you look at unit of work,.py, it hasn't changed in years.