Re: Howto handle inheritance with an abstract classes

[Thomas Dudziak <to...@gmail.com>]

Nils Liebelt wrote:

>Hi,
>
>I am struggling with a simple extend of an abstract class, trying to use
>multiple joined tables.
>
>I get following MetadataException: Can't create new base class object for
>'za.co.ibn.datasolutions.entities.Contact'
>
>A soon as I remove the abstract modifier everything works.
>
>Here's my Repository:
>
><class-descriptor
>    class="za.co.ibn.datasolutions.entities.Contact"
>    table="Contact"
>  
>
>    <field-descriptor
>        name="contactID"
>        column="contactID"
>        jdbc-type="INTEGER"
>        primarykey="true"
>        autoincrement="true"
>    >
>    </field-descriptor>
>    <field-descriptor
>        name="address"
>        column="address"
>        jdbc-type="VARCHAR"
>        length="254"
>    >
>    </field-descriptor>
></class-descriptor>
><class-descriptor
>    class="za.co.ibn.datasolutions.entities.Individual"
>    table="Individual"
>  
>
>    <field-descriptor
>        name="contactID"
>        column="contactID"
>        jdbc-type="INTEGER"
>        primarykey="true"
>        autoincrement="true"
>    >
>    </field-descriptor>
>    <reference-descriptor
>        name="super"
>        class-ref="za.co.ibn.datasolutions.entities.Contact"
>        auto-retrieve="true"
>        auto-update="true"
>        auto-delete="true"
>    >
>        <foreignkey field-ref="contactID"/>
>    </reference-descriptor>
></class-descriptor>
>
>It really don't know what else to try. Did everything according to the
>advanced technique tutorial. 
>  
>
The problem seems to be that for storing/materializing an instance of 
Individual, a concrete instance of the type referenced by the 
super-reference is needed. But since this class is abstract, no such 
object can be built. While this scenario may sound feasible in Java 
terms (because the fields of Contact are duplicated in Individual), when 
mapped to a reference OJB's behaviour actually makes sense because the 
mapping is a 'normal' reference. Only there is no concrete field to 
implement the reference on the Java side, but rather an anonymous field 
is used by OJB to store the end of the reference (an instance of 
Contact) and the values of this object are then used for the Individual 
instance.
So, I'd say there is currently no way around making Contact a concrete 
class.

Tom


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