$ cat test.py
from sqlalchemy import *

metadata = BoundMetaData('sqlite://')
metadata.engine.echo = True

class Test(object):
    pass

test_table = Table('test', metadata,
        Column('foo', Integer, primary_key=True),
        Column('bar', String)
)

m = mapper(Test, test_table, properties = {
        'bar': deferred(test_table.c.bar)
    }
)

metadata.create_all()

session = create_session(bind_to=metadata.engine)

t = Test()
t.foo = 42
session.save(t)
session.flush()

~


$ python test.py
[2006-07-11 19:24:55,278] [engine]: PRAGMA table_info(test)
[2006-07-11 19:24:55,278] [engine]: {}
[2006-07-11 19:24:55,280] [engine]:
CREATE TABLE test(
        foo INTEGER NOT NULL PRIMARY KEY,
        bar TEXT
)


[2006-07-11 19:24:55,280] [engine]: None
[2006-07-11 19:24:55,280] [engine]: COMMIT
[2006-07-11 19:24:55,282] [engine]: BEGIN
[2006-07-11 19:24:55,283] [engine]: SELECT test.bar AS test_bar
FROM test
WHERE test.foo = ?
[2006-07-11 19:24:55,283] [engine]: [42]
[2006-07-11 19:24:55,284] [engine]: ROLLBACK
Traceback (most recent call last):
  File "test.py", line 26, in ?
    session.flush()
  File "sqlalchemy/orm/session.py", line 233, in flush
  File "/home/alex/pydev/Sqlalchemy/trunk/sqlalchemy/orm/unitofwork.py", line 231, in flush
    trans.rollback()
  File "sqlalchemy/orm/session.py", line 55, in rollback
  File "sqlalchemy/engine/base.py", line 366, in rollback
  File "sqlalchemy/engine/base.py", line 211, in _rollback_impl
  File "sqlalchemy/engine/base.py", line 184, in _get_connection
sqlalchemy.exceptions.InvalidRequestError: This Connection is closed