芹菜和SQLAlchemy-此结果对象不返回行。它已自动关闭

Question

我有一个芹菜项目连接到MySQL数据库。其中一个表的定义如下：

class MyQueues(Base):
    __tablename__ = 'accepted_queues'

    id = sa.Column(sa.Integer, primary_key=True)
    customer = sa.Column(sa.String(length=50), nullable=False)
    accepted = sa.Column(sa.Boolean, default=True, nullable=False)
    denied = sa.Column(sa.Boolean, default=True, nullable=False)

而且，在我的设置中

THREADS = 4

我被困在code.py的函数中：

def load_accepted_queues(session, mode=None):

    #make query  
    pool = session.query(MyQueues.customer, MyQueues.accepted, MyQueues.denied)

    #filter conditions    
    if (mode == 'XXX'):
        pool = pool.filter_by(accepted=1)
    elif (mode == 'YYY'):
        pool = pool.filter_by(denied=1)
    elif (mode is None):
        pool = pool.filter(\
            sa.or_(MyQueues.accepted == 1, MyQueues.denied == 1)
            )

   #generate a dictionary with data
   for i in pool: #<---------- line 90 in the error
        l.update({i.customer: {'customer': i.customer, 'accepted': i.accepted, 'denied': i.denied}})

运行此命令时，出现错误：

[20130626 115343] Traceback (most recent call last):
  File "/home/me/code/processing/helpers.py", line 129, in wrapper
    ret_value = func(session, *args, **kwargs)
  File "/home/me/code/processing/test.py", line 90, in load_accepted_queues
    for i in pool: #generate a dictionary with data
  File "/home/me/envs/me/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2341, in instances
    fetch = cursor.fetchall()
  File "/home/me/envs/me/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 3205, in fetchall
    l = self.process_rows(self._fetchall_impl())
  File "/home/me/envs/me/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 3174, in _fetchall_impl
    self._non_result()
  File "/home/me/envs/me/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 3179, in _non_result
    "This result object does not return rows. "
ResourceClosedError: This result object does not return rows. It has been closed automatically

所以主要是部分

ResourceClosedError: This result object does not return rows. It has been closed automatically

有时也会出现这样的错误：

DBAPIError: (Error) (, AssertionError('Result length not requested length:\nExpected=1. Actual=0. Position: 21. Data Length: 21',)) 'SELECT accepted_queues.customer AS accepted_queues_customer, accepted_queues.accepted AS accepted_queues_accepted, accepted_queues.denied AS accepted_queues_denied \nFROM accepted_queues \nWHERE accepted_queues.accepted = %s OR accepted_queues.denied = %s' (1, 1)

我不能像处理大量数据时通常发生的那样正确地再现错误。我试图将THREADS = 4更改为1，错误消失了。无论如何，这不是一个解决方案，因为我需要将线程数保持在4。

另外，我对使用

for i in pool: #<---------- line 90 in the error

或者

for i in pool.all(): #<---------- line 90 in the error

找不到合适的解释。

总而言之：有什么建议可以跳过这些困难吗？

Answer 1

All together: any advise to skip these difficulties?

是的。您绝对不能同时在多个线程中使用会话（或与该会话相关联的任何对象）或连接，特别是对于DBAPI连接非常不安全的MySQL Python*。您必须对应用程序进行组织，使每个线程处理自己的专用MySQL-Python连接（因此与该会话相关联的SQLAlchemy连接/会话/对象），而不会泄漏到任何其他线程。

• 编辑：或者，您可以使用互斥锁将对会话/连接/DBAPI连接的访问限制为一次只访问其中一个线程，尽管这不太常见，因为所需的高度锁定往往会破坏首先使用多个线程的目的。