AW: [Objectbridge-developers] Re: OJB Technical Questions & Tutor ial #3

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

> -----Urspr=FCngliche Nachricht-----
> Von: Christian Sell [mailto:chr...@ne...]
> Gesendet: Mittwoch, 23. Mai 2001 00:33
> An: obj...@li...
> Betreff: [Objectbridge-developers] Re: OJB Technical Questions &
> Tutorial #3
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> Hello,
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> Hope you dont mind that I throw in my few cents here:
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> > > SUPPORT FOR MAPPING MANY-MANY RELATIONSHIPS
> > > Does OJB currently support the mapping of many-to-many
> > > relationships?  I
> > > didn't see any examples in the tests.  If not, is this=20
> planned for an
> > > upcoming release?
> > >
> > You are right, there is no support for m-n relationships.=20
> You have to
> > decompose it into two 1-n relationships. You have to do=20
> this for your
> RDBMS
> > ER model anyway, so its not such a big problem in my eyes.
> > There are some OJB users (marquier et al.) who are thinking=20
> of building an
> > n-m extension. But right now there are no concrete plans to=20
> include such a
> > feature.
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> I dont think the fact that in physical database modeling you=20
> decompose N-M
> relationships into separate 1-N relationships makes this a less =
useful
> (should I say crucial?) feature fo an O/R modeling tool.=20
> After all, it is
> the very purpose of the O/R tool to cover the technical=20
> details with a more
> abstract and intuitive layer. And even if I was going to=20
> program plain SQL,
> I would use a double join to directly access the final target=20
> entity and not
> load the rows of the associative table and then proceed from there.
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Sure, covering M-N with an O/R tool is an important feature.=20
The current development on OJB is mostly concerned with basic
infrastructure.
(Implementing ODMG, building a distributed architecture)
Things like
- automated generation of proxy classes
- automated support for M-N Relationships
- automated generation of mapping data and DLL from java classes
- automated reengineering of DDL to java classes and mapping data
- XMI interface
- GUI Mapping Workbench

are useful things (that's why commercial tools like toplink have them). =
But
they only make sense when the basic infrastructure is stable.=20
Of course I have no objections if some volunteers start working on M-N =
or
the like :-)

> > > ASSIGNING IDENTITY VALUES
> > > Some of the object graphs that I will be persisting have
> > > dependent/related
> > > objects which do not have IDs.  Do all objects have to=20
> have IDs when
> > > persisting using OJB?  If so, is there a way to have OJB=20
> transparently
> > > assign IDs using the SequenceManager, without the client code
> > > needing to
> > > call SequenceManager.getUniqueId() for each object?  If not,
> > > can I use a
> > > ConversionStrategy as a means of implementing transparent ID
> > > assignment?
> >
> > I'm not sure if I get you right: Your database table have=20
> no primary key
> > columns? Or do they have non numeric keys? Or do you have=20
> compound keys,
> > that contain of two or more columns?
> >
> > OJB does require to have primary keys. But they can be of=20
> arbitrary type
> > (not only INT) and may also be compound keys.
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> If I understand Sasha right, He is referring to what in E/R=20
> we used to call
> identifying relationships. This is where one entity is=20
> dependent on another
> in such a way that the foreign key becomes part of the=20
> primary key. If this
> is a 1-1 relationship, there is even no need for an=20
> additional identifier,
> therefore it should be sufficient to have a foreign key that=20
> also serves as
> primary key (the primary key is "inherited" by the dependent=20
> entity. In
> fact, if you implement inheritance on top of a relational=20
> model, this is
> exactly where you end up, dont you?). It would be rather=20
> inconvenient IMO if
> the dependent entity had to maintain an additional primary=20
> key just for the
> sake of the O/R mapping tool.
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> thanks, and good night,
> Christian
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Ah, OK, now I got it.=20
This of course possible. You just have to declare the foreign-key =
attribute
as primary key in the XML Mapping Repository.


Thomas

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