Fair enough. I had enough understanding of what must be going on to
know flush isn't straightforward, but I'm still glad I asked. Sorry for
having not read the documents very well and thanks for your answer,
because from it, I surmise that before_flush() *is* safe for session
operations, which is very good to understand more clearly.
Thanks.
On 1/26/2012 12:06 PM, Michael Bayer wrote:
On Jan 26, 2012, at 11:28 AM, Kent Bower wrote:
I think I understand why, during a flush(), if I use session.query().get() for
an item that was just added during this flush, I don't get the persistent
object I might expect because the session still has it as pending even though,
logically, it is already persistent.
I don't suppose you have any desire to support that, huh? The use case would
be related to the future ticket http://www.sqlalchemy.org/trac/ticket/1939 (and
http://www.sqlalchemy.org/trac/ticket/2350).
Attached is a script demonstrating the issue I've hit. I can work around it
with some difficulty, but I wanted your input and thoughts.
No, there's no plans to support this case at all; you're using the Session
inside of a mapper event, which is just not supported, and can never be due to
the nature of the unit of work. The most recent docstrings try to be very
explicit about this:
http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.MapperEvents.before_update
I guess I have to add session.query() and get() in there as well.
The way the flush works is not as straightforward as "persist object A; persist object B;
persist object C" - that is, these are not atomic operations inside the flush. It's more
like, "Perform step X for objects A, B, and C; perform step Y for objects A, B and C".
This is basically batching, and is necessary since it is vastly more efficient than atomically
completing each object one at a time. Also, some decisions are needed by Y which can't always be
made until X has completed for objects involved in dependencies.
A side effect of batching is that if we provide a hook that emits after X and
before Y, you're being exposed to the objects in an unusual state. Hence, the
hooks that are in the middle like that are only intended to emit SQL on the
given Connection; not to do anything ORM level beyond assigning column-based
values on the immediate object. As always, before_flush() is where ORM-level
manipulations are intended to be placed.
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