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

[Sascha A. K. <Ko...@un...>]

that sounds really good. When do you think you could have a first =
version ready?

An id for subtables is not a restriction, we would need that anyway if =
we don't want to store everything in the main table.

Thanks a lot
Sascha
  ----- Original Message -----=20
  From: David Forslund=20
  To: Mahler Thomas ; objectbridge=20
  Sent: Wednesday, January 31, 2001 5:28 PM
  Subject: Re: [Objectbridge-developers] RE: [objectbridge - Open =
Discussion] Polymorphism support?


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

  Thanks,

  Dave


    best regards,

    Thomas


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  David W. Forslund                               dw...@la...   =20
  Computer and Computational Sciences             =
http://www.acl.lanl.gov/~dwf
  Los Alamos National Laboratory          Los Alamos, NM 87545           =
=20
  505-665-1907                                    FAX: 505-665-4939