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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