Re: How do you map a reference to an interface-backed class? (non-trivial)

[Jakob Braeuchi]

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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