Hey Mike,
Thanks for taking the time to answer all of these questions! I think what
we're looking for is a way to perform some actions immediately before
"COMMIT" is sent to the database. The current solutions (as far as I can
understand them) all have some drawbacks:
*Perform actions in before_commit*
This event fires before a flush event, so the following code performs the
actions *before* the data has been flushed:
a = A()
a.name = 'newname'
db.session.commit()
*Mark a boolean flag in before_commit, and do actions in after_flush if the
boolean is set*
This fixes the above problem, but runs into a new issue:
a = A()
a.name = 'newname'
db.session.flush() # Boolean isn't set, so actions don't fire
db.session.commit() # There is no dirty state, so after_flush never gets
called here and actions don't fire
It looks like what we're looking for is a "immediately_before_commit_query"
hook. Does this exist in SQLAlchemy right now? If not, another alternative
we could go with is:
*Perform actions in before_commit, but call db.session.flush() first*
(suggested by Jonathan Vanasco earlier in this thread)
We manually call db.session.flush() to ensure that we don't have any
remaining "dirty" state, and in essence we can now perform actions
"immediately before the COMMIT query". It would look something like:
@event.listens_for(SignallingSession, 'before_commit')
def actions(...):
db.session.flush()
# Do other actions
I'm not immediately confident that this is equivalent to the conceptual
"immediately_before_commit_query" hook - mostly not sure if there are any
gotchas with this approach. Do you think this is a safe thing to do?
On Tuesday, September 19, 2017 at 1:46:23 PM UTC-7, Mike Bayer wrote:
>
> On Tue, Sep 19, 2017 at 4:06 PM, <mi...@benchling.com <javascript:>>
> wrote:
> > Just checking, but one scenario where "after flush before commit" would
> not
> > work is when all changes are flushed immediately before a commit, right?
> > In that case, "commit" would not automatically be preceded by a "flush"
> (and
> > the "after_flush_postexec" code would not run because no token from
> > "before_commit" has been set).
>
> if there is nothing to flush, like there's no objects or no attributes
> have been modified, then when commit() is called, it is usually the
> case that no flush occurs and you would not see these events.
>
>
> >
> > e.g.,
> > # make some changes
> > session.flush()
> > session.commit() # Does not kick off its own flush
> >
> > On Tuesday, September 19, 2017 at 6:27:12 AM UTC-7, Mike Bayer wrote:
> >>
> >> On Tue, Sep 19, 2017 at 12:19 AM, <mi...@benchling.com> wrote:
> >> > Followup question -- this the following order of events guaranteed?
> >> >
> >> > before_commit --> after_flush --> after_flush_post_exec
> >> >
> >> > i.e., is an after_flush_post_exec guaranteed to run after the
> >> > before_commit
> >> > preceding a commit?
> >>
> >> if the flush didn't raise then yes.
> >>
> >>
> >> >
> >> > --
> >> > SQLAlchemy -
> >> > The Python SQL Toolkit and Object Relational Mapper
> >> >
> >> > http://www.sqlalchemy.org/
> >> >
> >> > To post example code, please provide an MCVE: Minimal, Complete, and
> >> > Verifiable Example. See http://stackoverflow.com/help/mcve for a
> full
> >> > description.
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> > --
> > SQLAlchemy -
> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
> >
> > To post example code, please provide an MCVE: Minimal, Complete, and
> > Verifiable Example. See http://stackoverflow.com/help/mcve for a full
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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