How does it work when a class has been changed ?

Hi,

In OODBMS, class-based OO, suppose I save an object, then later modify
its class by removing/adding fields. Does it mean I cannot load the
old object ? How OODBMS handles this ?

Thank you

In the GemStone OODBMS, classes are first class objects. So when you
"modify" the class you are really creating a new version of the class. The
old instances are still instances of the old class until they are migrated
to the new version. And you can have instances of the old and new versions
of the class at the same time, although most people don't do this. In
practice, most people migrate all the old instances to instances of the new
version of the class.

Norm Green


"romerun" wrote in message
news:1194027105.434913.6020-at-o80g2000hse.googlegrou ps.com...
> Hi,
>
> In OODBMS, class-based OO, suppose I save an object, then later modify
> its class by removing/adding fields. Does it mean I cannot load the
> old object ? How OODBMS handles this ?
>
> Thank you
>

In Intersystems Caché you can still load any existing objects but you
won't have access to any properties which have been removed from the
class definition (though the data is still there and you can get to it
via direct access to the underlying multi-dimensional Database engine) while
new properties simply won't have a value.

Wolf Koelling

On Nov 2, 6:11 pm, romerun wrote:
> Hi,
>
> In OODBMS, class-based OO, suppose I save an object, then later modify
> its class by removing/adding fields. Does it mean I cannot load the
> old object ? How OODBMS handles this ?
>
> Thank you