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From Jakob Braeuchi <jbraeu...@gmx.ch>
Subject Re: How do you map a reference to an interface-backed class? (non-trivial)
Date Wed, 23 Jun 2004 16:55:20 GMT
hi thomas, andrew,

i recently came across your discussion about mapping interfaces. the approach 
using a class-name-field is imo very interesting. i assume that the mapped 
classes only share a common interface and are not in the same extent-hierarchy 
in the repository. otherwise you would have a problem because ojb requires the 
pk to be unique within a hierarchy.

i think you should open a feature request in scarab so the idea does not get lost.

thanks
jakob

Thomas Dudziak wrote:

> Clute, Andrew wrote:
> 
>> That's what I assumed, and was afraid of.
>>
>> I think I can get around it temporarily by actually creating a composite
>> field that contains the classname and the PK in the same field (i.e.
>> "foo.bar.CatalogItem:12345"), and then creating a custom Conversion that
>> will take that string and instantiate the object.
>>
>> Now, is there any technical reason why there isn't a strategy to map a
>> class that has a reference to an interface-backed class where the
>> concrete classes are not mapped in the same table? It would seem to me,
>> that such a feature could be implemented pretty straight-forward in the
>> current architecture (and I would be willing to take that on), but I
>> want to make sure I understand all of the ramifications of why this
>> isn't done at this time (feature never implemented?, won't work with
>> what we currently have?, etc.
>>
>> One staight-forward approach would to modify the repostiroy_user.xml
>> definitions to allow the following type of mapping:
>>
>>    <reference-descriptor
>>        name="interfaceItem"
>>        class-name-field-ref="interfaceClassName"
>>    >
>>        <foreignkey field-ref="interfaceGuid"/>
>>    </reference-descriptor>
>>
>> <field-descriptor
>>        name="interfaceClassName"
>>        column="interface_class_name"
>>        jdbc-type="VARCHAR"
>>    />
>> <field-descriptor
>>        name="interfaceGuid"
>>        column="interface_guid"
>>        jdbc-type="VARCHAR"
>>        access="anonymous"
>>    />
>>
>>  
>>
> That would work, I guess, though I'd rather put the class name next to 
> the foreignkey:
> 
>    <reference-descriptor
>        name="interfaceItem"
>    >
>        <concreteClassName field-ref="interfaceClassName"/>
>        <foreignkey field-ref="interfaceGuid"/>
>    </reference-descriptor>
> 
> IMO this concept should be extended to collections as well 
> (inverse-concreteClassName) to get rid of the hard-coded 
> 'ojbConcreteClass' stuff. Might be a good use of anonymous fields, too.
> 
> And sure, if you want to have a go at it, by all means go ahead !
> 
> Tom
> 
> 
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