django transaction.commit在成功时不回滚事务

我尝试在mysql上使用django事务 the commit_on_success decorator . 根据文档,“如果函数引发异常,Django将回滚事务。”但是,这似乎对我不起作用:

>>> @transaction.commit_on_success
... def fails():
...     Site.objects.create(name="New Site", ip_address="127.0.0.1")
...     raise ValueError("oh noes!")
... 
>>> Site.objects.count()   
2
>>> fails()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.6/site-packages/django/db/transaction.py", line 240, in _commit_on_success
    res = func(*args, **kw)
  File "<stdin>", line 4, in fails
ValueError: oh noes!
>>> Site.objects.count()
3
>>>

我很确定MySQL支持事务处理;我需要使用不同的表类型还是其他类型?