[ZODB-Dev] Migrating classes in ZODB 3.5.1

[Jeremy Hylton]

On 11/9/05, David Binger <dbinger at mems-exchange.org> wrote:
>
> On Nov 9, 2005, at 9:21 AM, Syver Enstad wrote:
>
> >>> What I want to do is just update the persistent instance so that
> >>> it will
> >>> load from the new module/ new class after I have done a one time
> >>> update
> >>> of the database.
> >>>
> >>
> >> I fear this will not work.
> >>
> >> For efficiency reasons, the class designator (usually a
> >> module, classname tuple) is stored both in the object itself
> >> and in persistent references. While you will be able
> >> to modify the object's class the persistent references will
> >> only change when the containers are stored into the ZODB.
>
> Is there something to prevent your update script from
> setting _p_changed = 1 on every persistent object that contains
> a reference to an instance of the moved class?
> I think that if you do that and commit and pack the database, the
> old module/class references will be purged.
>
> If you do this, the sys.modules hacking only has to happen
> in the update script.

How do you find all the references?  It seems like you would need to
execute a transaction that iterated over every object in the storage
and searched the pickle for references to the class.

I can imagine the update transaction taking a really long time to run,
which increases the possibility that it will get a conflict error. 
Maybe it's a resolvable conflict, but not using the current mechanism.

Jeremy