Re: [Objectbridge-developers] RE: [objectbridge - Open Discussion] Polymorphism support?

[David F. <dw...@la...>]

At 04:41 PM 1/31/2001 +0100, Mahler Thomas wrote:
>Hi Sascha,
>
> > -----Urspr=FCngliche Nachricht-----
> > Von: nobody [mailto:no...@so...]
> > Gesendet: Dienstag, 30. Januar 2001 22:31
> > An: no...@so...
> > Betreff: [objectbridge - Open Discussion] Polymorphism support?
> >
> >
> > Read and respond to this message at:
> > http://sourceforge.net/forum/message.php?msg_id=3D103071
> > By: koenig
> >
> > Hi,
> >
> > I looked into ObjectBridge as a O/R mapping tool for our OpenEMed=
 project
> > (http://telemed.lanl.gov/OpenEMed).
> >
>Thanks for your interest! It would be a great opportunity to proof the
>usability of OJB if it could solve your persistence needs.
>
> > Unfortunately I didn't found a mechanism for supporting polymorph
> > objects.
>
>You are right. As of today there is no support for Polymorphism.
>But of course it is an essential feature. It is also required by the ODMG
>spec.
>If it is a feature you need urgently I can start to work on it immediately.
>
>I have already a design in mind, so it will take only a few days...

We urgently need this feature.  Our only other option in open source is=
 castor.

> >
> > In particular:
> > Our main class has a field 'value' of an interface type. This
> > interface type
> > is implemented by several concrete classes that get stored in the
> > 'value' field
> > of the main object.
> > I'd like to store these objects in separate tables referenced
> > from the main
> > object, but even storing them as embedded objects would be fine.
> >
> > Is there a way to do this with ObjectBridge?
>
>As long as I'm not finished with a real solution, you could try the
>following:
>you can use an additional field 'valueOID' of type String which takes a
>serialized
>representation of the concrete values Identity:
>valueOID =3D new Identity(value).serialize();
>instead of making the field value persistent you store the serialized OID=
 in
>a db column.
>Of course your value field is then transient. On loading your Object you
>have to reconstruct it e.g. in a constructor by:
>value =3D (YourInterface)
>broker.getObjectByIdentity(Identity.forName(valueOID));
>
>I'm using this approach for untyped Collections (like org.odmg.DList) and=
 it
>works quite well. But of course it's not type safe and brings one=
 additional
>level of indirection into the mapping...
>
>Of course you can also store complete serialized "embedded" objects in a
>single db column, but this is even worse from a design point of view...

The difficulty with what you suggest is that we are trying to do all of=20
this through
an interface that lets us talk to multiple database sources.   So we need a=
=20
"generic" solution
that we don't have to wire much into our objects.  We are willing to do=20
some specializations (such as
implementing the Persistent interface), but don't like to have to get into=
=20
the details of the objects much.

Thanks,

Dave

>best regards,
>
>Thomas
>
>
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David W. Forslund                               dw...@la...
Computer and Computational Sciences             http://www.acl.lanl.gov/~dwf
Los Alamos National Laboratory          Los Alamos, NM 87545
505-665-1907                                    FAX: 505-665-4939