[ZODB-Dev] Schema migration, rename class

[Jeremy Hylton]

On Wed, 2004-03-31 at 09:19, Syver Enstad wrote:
> I am looking for good resources on schema migration with ZODB. 
> 
> http://www-106.ibm.com/developerworks/linux/library/l-pypers.html
> 
> Shows some of the techniques (the article is about Pickling) The
> article describes how to rename a class by changing the self.__class__
> to the renamed class in __setstate__ but this doesn't seem to work.

(I haven't followed the link yet, but ...)  You can't change the class
of a persistent object, although it may seem like you can because you
can assign to __class__.  The problem is that persistent references
contain the class and oid of the object.  So you can change __class__,
but the next time the object is loaded it will get whatever class its
referrer thinks it has.

I don't know if we have a good answer for changing the class.  My first
guess is that you'd need to write a tool that scanned the entire
database.  You could either do a low-level (pickle-level) translator
against the storage or do an application-level translator that loaded
each object and marked it as changed if it referred to the object that
was changing class.  Neither sounds very appealing.

Jeremy