SQLAlchemy，获取未绑定到会话的对象

我试图从数据库中获取一组对象，并将其传递给未连接到数据库的另一个进程。我的代码看起来像下面的一个，但我不断得到:

sqlalchemy.exc.UnboundExecutionError: Instance <MyClass at 0x8db7fec> is not bound to a Session; attribute refresh operation cannot proceed

当我尝试在 get_list()方法之外查看列表中的元素时。

def get_list (obj):
sesson = Session()
lst = session.query(MyClass).all()
session.close()
return lst

但是，如果我使用这个:

def get_list_bis (obj)
session = Session()
return session.query(MyClass).all()

我能够使用元素，但是担心会话的状态，因为它没有关闭。

我错过了什么？

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最佳答案

If you want a bunch of objects produced by querying a session to be usable outside the scope of the session, you need to expunge them for the session.

In your first function example, you will need to add a line:

session.expunge_all()

before

session.close()

More generally, let's say the session is not closed right away, like in the first example. Perhaps this is a session that is kept active during entire duration of a web request or something like that. In such cases, you don't want to do expunge_all. You will want to be more surgical:

for item in lst:
session.expunge(item)
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In my case, I was saving a related entity as well, and this recipe helped me to refresh all instances within a session, leveraging the fact that Session is iterable:

map(session.refresh, iter(session))  # call refresh() on every instance

This is extremely ineffective, but works. Should be fine for unit-tests.

Final note: in Python3 map() is a generator and won't do anything. Use real loops of list comprehensions

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This often happens due to objects being in expired state, objects get expired for example after committing, then when such expired objects are about to get used the ORM tries to refresh them, but this cannot be done when objects are detached from session (e.g. because that session was closed). This behavior can be managed by creating session with expire_on_commit=False param.

>>> from sqlalchemy import inspect
>>> insp = inspect(my_object)
>>> insp.expired
True  # then it will be refreshed...

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