On Aug 30, 2012, at 10:32 AM, Tony Moutaux wrote:
> Hi there !
>
> I'm going mad looking for a solution for what seems a simple problem.
>
> When object A is updated, I also want object B to be updated, using some
> computation based on A new values.
> From now, I can detect when A is modified using after_update.
>
> What I try is to look at event Session.after_flush, if in Session.dirty there
> is an instance of A, I call the update of B. Within after_flush I can see
> Session.dirty with the modified B. But no SQL UPDATE is issued at all.
>
> Any tip for me ?
You do this kind of thing most simply within the objects themselves, not as
much in Session events, depending on how things are organized. Such as in the
__init__() method of A, or by using an event listener on an attribute of A.
Only if "some computation based on A new values" means that there are SQL-level
functions which you need to get at, does the rationale for
"after_update"/"after_flush" events to come up. Unfortunately you can't emit
a flush() within any of these events, as they are already local to the flush()
call itself. Nor can you change the state of the Session within any of the
"after_" events, as the Session is already doing its work. You can add new
state to the Session within the "after_flush_postexec()" method, however you'd
need to call flush() again for those changes to be flushed.
So the options:
1. if "some computation" is only "in-Python" computation, and *not* SQL
functions, then set the state of B within object-level events, such as the
__init__() of A, a @property on A, @validates events on A, or attribute events
on A.
2.if "some computation" involves SQL functions:
a. emit your modifications to B using only SQL statements and
Session.execute(), not by adding any new state to the Session or re-flushing.
b. pre-execute your SQL functions for A *outside* of the flush, then apply
values to B as needed.
c. use a second Session inside the event:
def after_flush(session, flush_context):
my_bs = figure_out_bs(session)
new_session = Session(bind=session.connection())
new_session.add_all(my_bs)
# this won't actually COMMIT the transaction as we are
# already inside one
new_session.commit()
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