Based on my observations it happens only with concurent inserts/ updates. One thread :

{{{ f_table.insert().execute() session.flush() transaction = session.begin() nested = session.begin_nested() try: f_table.insert().execute() except IntegrityError: #record is exists and we got exception corresponding to contraint stuff = session.query(Path).select_from(..) ..update.. nested.commit()

transaction.commit() session.clear() }}}

Another thread: {{{ f_table.delete(...) #or update }}}