I had forgotten the danger of populate_existing() also because we make
it automatically issue a flush() for these purposes. Documentation note
sounds good.
On 3/5/2013 10:45 AM, Michael Bayer wrote:
populate_existing() blows away any pending changes on the object so
turning it on by default would be a surprise in a lot of cases.
typically if someone is working with FOR UPDATE they're already
programming a very specific section in a very careful manner, it's not
something that's done casually. I would think that it would be used to
select a row right at the start of a transaction, that is definitely not
already in the Session.
in my own experience, any time I've actually tried to work with
pessimistic locking my application ends up deadlocking at 2 AM, so I
can't say I have the background to really say how this method should be
used. At this point, a documentation note is certainly something doable
(because I would hope very much that anyone using this method has read
the documentation carefully).
On Mar 5, 2013, at 10:00 AM, Kent <jkentbo...@gmail.com
<mailto:jkentbo...@gmail.com>> wrote:
By design, when a query() fetches an existing object, it doesn't
refresh the values unless populate_existing() is included with the
query. The documentation for populate_existing() states it isn't
meant for general purpose.
Occasionally, however, objects need to be selected FOR UPDATE,
with_lockmode('update'), to guarantee against timing problems with
concurrent database users, particularly when the record fetched is
used as a base for the update. For example, if I need to update a
record's quantity field by a delta of +5, it is extremely important
that I have the most recent Object.quantity datum; truly, that was the
reason I used with_lockmode('update') in the first place.
I get uneasy to think that the user needs to remember to invoke
populate_existing() when selecting a record FOR UPDATE to guard
against the possibility that there is a stale version of the instance
in the session.
I will likely add that to our framework's Query subclass, but thought
you might consider the implications here. Generally, sqla is extremely
optimistic regarding locks, but in the event when the user is
specifying with_lockmode('update'), we've left the realm of optimistic
locking and entered pessimistic, so it seems reasonable to consider
automatically enforcing populate_existing() when
with_lockmode('update') is used?
Something to consider; I'd be interested in your thoughts.
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