On Oct 3, 2010, at 11:09 PM, Graham Lowe wrote:
>
>>>> [(t.i, t.s) for t in session.query(T)]
> [(1, u'a'), (2, u'b')]
>>>> session.identity_map
> {(<class '__main__.T'>, (2,)): <sqlalchemy.orm.state.InstanceState
> object at 0x1c47a90>}
as your list comprehension completes, only the most recent value remains
strongly referenced. Python garbage collection has kicked in and cleaned out
the first T instance, only the second remains.
> The identity_map is holding onto the tail of this list comprehension.
> This leads some really weird results:
>
> In psql:
> update t set s='c';
>
>>>> [(t.i, t.s) for t in session.query(T)]
> [(1, u'c'), (2, u'b')]
>
> As an end-user, I would either expect both items to be updated or
> neither of them to be updated.
You should expect only that which is strongly referenced outside of the scope
of the session, or has pending changes established upon it before being
dereferenced, to remain in the identity map. Objects that lose their
reference are garbage collected, and the Session cleans out its accounting for
otherwise unchanged items when weakref callbacks alert it to this activity.
This is how a single session can watch hundreds of thousands of persistent
objects go by without the memory size of the application growing any more than
the size of collections maintained outside of the session, assuming the session
is regularly flushed (hence autoflush=True the default in that regard).
If you weren't using autocommit=True, a transaction would remain open
throughout the scope of your working with T objects in the session. If you
were to then use SERIALIZABLE transaction with PG, this would prevent the
inconsistent read from occurring. Allowing transaction isolation to be useful
is why autocommit=False is the default and we don't talk much about
autocommit=True. Once you were to call session.rollback() or session.commit(),
the state of each T still in memory would be expired and you'd see the 'c'
value for both T objects after you re-access them in the subsequent transaction
that starts up the moment you query again. In this way, the Session doesn't
have to reinvent transactional concurrency behavior, or make continuous
expensive and complicated guesses as to when it should expire which attributes
- it very simply assumes the greatest data consistency with a SERIALIZABLE
transaction, and has only simple decisions to be made regarding attribute
state. That doesn't mean you have to use SERIALIZABLE. You'd use it
typically if you are expecting to deal with individual rows in a concurrent
fashion. Otherwise the default of READ COMMITTED is usually fine for most
needs.
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