objectbridge-developers

[Objectbridge-developers] optional relationships

[Dirk O. <di...@xa...>]

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?)

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. 

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:

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

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

Re: [Objectbridge-developers] optional relationships

[Dirk O. <di...@xa...>]

David Forslund wrote:

> > 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.

Good to hear that. But how do you do the mapping? I currently have no
idea how to accomplish that without a foreign key. Or do you put code
into Trait to retrieve the parent?

-dirk

Re: [Objectbridge-developers] optional relationships

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

At 10:55 PM 8/9/2001, Dirk Olmes wrote:
>David Forslund wrote:
>
> > > 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.
>
>Good to hear that. But how do you do the mapping? I currently have no
>idea how to accomplish that without a foreign key. Or do you put code
>into Trait to retrieve the parent?

The Trait doesn't need to retrieve the parent (but it can return the id of 
its parent if it needs to be retrieved),
but I do have code for the parent to retrieve the child if it isn't 
present.  I think the OJB examples do not have foreign keys, 
either.  Several DBMS' I deal with don't handle foreign keys correctly, anyway.


Dave


>-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