如何判断 SQLAlchemy 中发生回滚的原因？

Question

当我使用 SQLAlchemy 插入 MySQL 时，我看到开发服务器的输出日志中出现消息“ROLLBACK”。我如何知道为什么会发生回滚？

2011-04-10 00:35:32,736 INFO  [sqlalchemy.engine.base.Engine.0x...4710][MainThread] INSERT INTO pageview (time, unit_id, visitor_id, url_id, referrer_id, reservation_id, visit_id) VALUES (%s, %s, %s, %s, %s, %s, %s)
2011-04-10 00:35:32,736 INFO  [sqlalchemy.engine.base.Engine.0x...4710][MainThread] (datetime.datetime(2011, 1, 31, 0, 1, 53), 120L, 5538L, 11075L, 11076L, 5538L, None) 
2011-04-10 00:35:32,737 INFO  [sqlalchemy.engine.base.Engine.0x...4710][MainThread] ROLLBACK 
Starting server in PID 10158. 
serving on 0.0.0.0:6543 view at http://127.0.0.1:6543

我注意到字母“L”出现在每个外键值之后（例如：unit_id 字段是“120L”而不是“120”）。这可能与这个问题有关吗？

这是执行插入的 python 代码：

@classmethod
def unconverted(class_):
    session = DBSession()
    return session.query(class_).filter(class_.pageview == None).order_by(class_.time).limit(5).all()

@classmethod
def convert_all(class_):
    session = DBSession()

    unconverted = class_.unconverted()
    for item in unconverted:

        pageview = PageView(raw_request=item)
        item.pageview = pageview
        session.add(item)

    session.flush()
    transaction.commit()
    session.close()

编辑：

按照 plaes 的建议，在 session.flush() 上执行 try/ except 时出现以下异常：

2011-04-10 11:33:44,462 INFO  [sqlalchemy.engine.base.Engine.0x...3750][MainThread] ROLLBACK
(IntegrityError) (1452, 'Cannot add or update a child row: a foreign key constraint fails (`metrics`.`pageview`, CONSTRAINT `pageview_ibfk_1` FOREIGN KEY (`unit_id`) REFERENCES `unit` (`id`))') 'INSERT INTO pageview (time, unit_id, visitor_id, url_id, referrer_id, reservation_id, visit_id) VALUES (%s, %s, %s, %s, %s, %s, %s)' (datetime.datetime(2011, 1, 31, 0, 1, 53), 120L, 5608L, 11215L, 11216L, 5608L, None)

发生了什么错误？

Answer 1

当异常发生时，你总是可以打印出错误：

try:
    transaction.commit()
except Exception, e:
    session.rollback()
    print str(e)