I'm not sure how OJB does it, but that would either mean a lot of queries,
or an awful lot of outer joins!
I wouldn't of thought there was a way to tell OJB to pick tables based on
their foreign id. Generally you use references to exact types, or supertypes
that are extended by a few classes.
I think the correct way to deal with this is by using extents. I say this
because in the java class that references another object, you will have to
use the same approach.
Lets say you have a classes A,B,C,D, etc
If in A it has a reference to myFriend: as you don't know the type of
myFriend (could be A, B, C, D, etc), you can only refer to something that
all these classes extend (or implement).
So,
class A extends BaseClass {
BaseClass myFriend;
}
and all the classes muse extend BaseClass.
I'm not sure what it is you're trying to map, but I've never come across a
case where you always need to refer to any class.
You could use extents like this, and use this where you need to reference
*any* object, but you could still reference a type directly where you know
what it will be.
So if you know that myFriendFKID will always be a B:
class A extends BaseClass {
B myFriend;
}
You can just use a reference descriptor in the repository referencing the
type B, not the BaseClass extent.
Hope this clarifies things,
Daniel.
> -----Original Message-----
> From: news [mailto:[EMAIL PROTECTED] Behalf Of Sebastian
> Sent: 02 June 2004 17:47
> To: [EMAIL PROTECTED]
> Subject: Re: Mapping question
>
>
> Hi Daniel,
>
> I had a look at the extents now. I think this will basically work. The
> problem I see is, that the database has 110 tables and all are using a
> globally generated OID. In my basic class I would have to list all other
> classes as extents of this one.
>
> What does this mean for the performance of OJB? will OJB now query all
> 110 tables to find the right object?
>
> Since I know quite easily based on the first 3 letters of the OID what
> type it is, can I somehow tell that OJB as a hint to search the
> right table?
>
> Sebastian
>
> Daniel Perry wrote:
>
> > What about using an extent and having both types extend it. It
> would pick
> > the right class based on whichever table has an item with that
> primary key.
> >
> > Daniel.
> >
> >
> >>-----Original Message-----
> >>From: news [mailto:[EMAIL PROTECTED] Behalf Of Sebastian
> >>Sent: 02 June 2004 17:07
> >>To: [EMAIL PROTECTED]
> >>Subject: Re: Mapping question
> >>
> >>
> >>To simplify my question:
> >>
> >>I have a table containing a foreign key column and the foreign key of a
> >>row points to different tables based on the first three letters of the
> >>foreign key. E.g. when it starts with "art" then it points to a record
> >>in the article table. When it starts with "cat" then it points to a
> >>category table.
> >>
> >>This means OJB should based on the first three letters of the foreign
> >>key know what kind of object to reference.
> >>
> >>What kind of custom mapper or converter or whatever do I have to write.
> >>
> >>Thanks,
> >>Sebastian
> >>
> >>
> >>Sebastian wrote:
> >>
> >>>Hi,
> >>>I'm trying to map some tables with OJBs but don't know how to express
> >>>their relation. Maybe someone can give me a hint.
> >>>
> >>>I have three tables:
> >>>articles with OID as primary key
> >>>categories with OID as primary key
> >>>custom_fields with OID as primary key, OID_FK as foreign key
> >>>
> >>>OID is generated uniquely over all tables in a format: "first 3 letters
> >>>of the table name" plus a "incrementing value". E.g.
> >>>cat01,art02,art03,cus04.
> >>>
> >>>An article as well as a category can have a custom field that is stored
> >>>in the custom_fields table, the OID_FK is the OID of the
> >>
> >>related article
> >>
> >>>or category. The first three letters of OID indicate the "object type"
> >>>article or category.
> >>>
> >>>How do I have to configure the repository.xml so that always the right
> >>>object (a category or an article) is referenced by an
> >>
> >>customField object?
> >>
> >>>Thanks in advance,
> >>>Sebastian
>
>
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