AW: [Objectbridge-developers] Q: How to setup the databas e?

[Mahler T. <tho...@it...>]

Hallo Christian!

> -----Urspr=FCngliche Nachricht-----
> Von: Christian Sell [mailto:chr...@ne...]
> Gesendet: Mittwoch, 8. August 2001 18:54
> An: Mahler Thomas
> Betreff: Re: [Objectbridge-developers] Q: How to setup the databas e?
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> Hi Thomas and all,
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> doe these mapping variations support polymorphism (sorry if this is
> mentioned in the docs)? I.e., if I select A's via OQL, do I=20
> get B's and C's
> as well - correctly instantiated, of course.
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Yes, Polymorphism is fully supported. That is if you select for A's =
with
some criteria you will get all A's, B's and C's that match this =
criteria.
Polymorphism works also with interfaces. That if A implements interface =
IA
(and B and C extending A) and you have a query on IA with some criteria =
you
will get all A's, B's and C's.
All objects will be properly instantiated. There is a sample for this
feature in test.ojb.broker with classes InterfaceArticle, Article,
BookArticle and CdArticle demonstrating the respective mapping =
techniques.


> Option 3 seems like the most obvious mapping variation for=20
> inheritance - any
> plans to support it soon?
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I don't agree on that. All three variants may be appropiate solutions =
to
model a class hierarchy. I had a lot of discussions with experienced
software developers and DBA's and none of them ever used Option three. =
All
people I spoke to agreed that this variant is not as performant as the
others as it needs a join about at least two tables for retrieving even =
the
simplest object of a derived class.

OJB allows you to work with such a database schema if neccessary: You =
have
to define a 1-1 mapping between tables A and B (and A and C) and you =
will
need some kind of wrapper object that maintains the 1-1 relationship.
That's not ideal but it works.

Until now there have been no requests to implement the "Option 3", thus =
I
have currently no plans to implement it.
(nevertheless, volunteers to do this job are always welcome :-) )

regards,

Thomas


> ----- Original Message -----
> From: "Mahler Thomas" <tho...@it...>
> To: "'Ivan Toshkov'" <to...@cr...>; "Mahler Thomas"
> <tho...@it...>
> Cc: "Objectbridge (E-Mail)"=20
> <obj...@li...>
> Sent: Wednesday, August 08, 2001 4:53 PM
> Subject: AW: AW: AW: AW: [Objectbridge-developers] Q: How to setup =
the
> databas e?
>=20
>=20
>=20
>=20
> > -----Urspr=FCngliche Nachricht-----
> > Von: Ivan Toshkov [mailto:to...@cr...]
> > Gesendet: Mittwoch, 8. August 2001 16:33
> > An: Mahler Thomas
> > Betreff: Re: AW: AW: AW: [Objectbridge-developers] Q: How=20
> to setup the
> > databas e?
> >
> >
> > Hi again,
> >
> > I couldn't find an example of using OJB with an hierarchy=20
> of classes.
> > The only exception is test.ojb.broker.Article and
> > test.ojb.broker.BookArticle, but the latter does not add any
> > new fields,
> > so it's not exactly what I'm looking for.
> >
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> It does! BookArticle has two additional attributes:
> /** books author*/
> private String author;
> /** ISBN No of Book*/
> private String isbn;
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> (VisualAge does not keep all attributes at the top of a class=20
> definition. I
> guess that's why you did not find those attributes.)
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> There is also the class CdArticle that does not extend Article but
> implements the interface Article and introduces also two more=20
> attributes
> than Article.
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> > To be more specific, let's say we have the following classes:
> >
> > abstract class A
> > {
> >   int num;
> > }
> >
> > class B extends A
> > {
> >   String name;
> > }
> >
> > class C extends A
> > {
> >   int count;
> > }
> >
> > The theory says, there are generally 3 representations:
> >
> > 1. Store the whole hierarchy in one table:
> >
> > table A:
> > +-----+-----+------+-------+------+
> > | oid | num | name | count | type |
> > +-----+-----+------+-------+------+
> >
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> supported by OJB. In the ClassDescriptor of each class you=20
> just have to
> declare that it maps to table A.
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> > 2. Store each concrete class in separate table:
> >
> > table B:
> > +-----+-----+------+
> > | oid | num | name |
> > +-----+-----+------+
> >
> > table C:
> > +-----+-----+-------+
> > | oid | num | count |
> > +-----+-----+-------+
> >
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> supported by OJB.
> In the ClassDescriptor for class B you have to declare that=20
> it maps to table
> B (same for Class C and table C).
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> > 3. Store each class in a separate table:
> >
> > table A:
> > +-----+-----+
> > | oid | num |
> > +-----+-----+
> >
> > table B:
> > +-----+------+
> > | oid | name |
> > +-----+------+
> >
> > table C:
> > +-----+-------+
> > | oid | count |
> > +-----+-------+
> >
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> not directly supported. Could be relaized via a foreign key relation.
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> HTH,
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> Thomas
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> > Which of these are currently supported by OJB and how do the XML
> > description looks like?
> >
> > Best regards,
> > Ivan
> >
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