Using SAVEPOINT

SAVEPOINT transactions, if supported by the underlying engine, may bedelineated using the begin_nested()method:

Session = sessionmaker()
session = Session()
session.add(u1)
session.add(u2)

session.begin_nested() # establish a savepoint
session.add(u3)
session.rollback()  # rolls back u3, keeps u1 and u2
 


session.commit() # commits u1 and u2

begin_nested() may be called any numberof times, which will issue a new SAVEPOINT with a unique identifier for eachcall. For each begin_nested() call, acorresponding rollback() orcommit() must be issued. (But note that if the return value isused as a context manager, i.e. in a with-statement, then this rollback/commitis issued by the context manager upon exiting the context, and so should not beadded explicitly.)

When begin_nested() is called, aflush() is unconditionally issued(regardless of the autoflush setting). This is so that when arollback() occurs, the full state of thesession is expired, thus causing all subsequent attribute/instance access toreference the full state of the Session rightbefore begin_nested()

begin_nested(), in the same manner as the less oftenused begin() method, returns a SessionTransaction objectwhich works as a context manager.It can be succinctly used around individual record inserts in order to catchthings like unique constraint exceptions:

for record in records:
    try:
        with session.begin_nested():
            session.merge(record)
    except:
        print("Skipped record %s" % record)
session.commit()