Re: [Objectbridge-developers] optional relationships

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

At 08:51 PM 8/9/2001, Dirk Olmes wrote:
>Hi,
>
>I just started using ObjectBridge for a small hobby project. I use it
>with the FrontBase dbms which had me digging around in the source a bit
>to make it work (wasn't hard). Now I found a problem which I could
>easily work around but since tihs is a common modeling case I thought of
>a general solution.
>
>To illustrate things better I'll just give a short description of my
>tables: I have a table 'Person' which has a to-many relationship to the
>table 'Folder'. The 'Folder' table will later have a reflexive
>relationship to itself (so I can have subfolders to every folder). (BTW:
>is that possible with ObjectBridge right now?)

Yes, it does.   We are using it this way now.  One of my applications has
the same structure.  It has an "Identity: object which contains many "Traits"
Traits can contain other Traits.  I have two tables Identity and Trait


>In order to model that, the 'Folder' table needs a foreign key which can
>be NULL for the case that a folder has no parent folder.

In my case I don't allow for a Trait to exist by itself, so a Trait has a Trait
or an Identity as a parent.   I don't formally use a foreign key.


>This empty foreign key already throws an exception when trying to
>resolve the relationship from User to Folder. Since JdbcAccess tries to
>put values into the Folder instance using reflection it stumbles when
>trying to set the null foreign key.
>
>While I could work around this giving the Folder a special foreign key
>(e.g. 0) for the no-parent-folder case I think such cases should be
>handled by the framework. Currently, I could think of two possible
>solutions to the problem:

We have another application that handles the case as you indicate for the
"top of the tree"

Our objects can be handled by OJB or other Object-Relational mapping tools
as well as directly by an OODBMS.   I don't like to have a method like "2)"
unless it has some other benefit to the object.

I hope this helps a little,

Dave


>1) extend the mapping to let the user specify which value object he
>wants to have assigned for individual attributes. So there would be a
><field.valueType/> in addition <field.name/> node in the mapping XML
>file. This would allow for e.g. Integer values to be passed. To make
>this work with reflection, JdbcAccess should use a set method if that's
>available (could be found out using reflection)
>
>2) define an interface with methods that could let the instance itself
>decide what to do in such cases. E.g. we could have a method named
>"unableToTakeNullForField(String fieldName)".
>
>Does anyone have a better idea?
>
>-dirk
>
>_______________________________________________
>Objectbridge-developers mailing list
>Obj...@li...
>http://lists.sourceforge.net/lists/listinfo/objectbridge-developers

David W. Forslund                               dw...@la...
Computer and Computational Sciences             http://www.acl.lanl.gov/~dwf
Los Alamos National Laboratory          Los Alamos, NM 87545
505-663-5218                                    FAX: 505-665-4939