Extents of Interfaces without factory methods

[Brian Latimer <bl...@apcc.aspca.org>]

I'm attempting to query for all extents of an interface but I don't have a 
factory method to create a default instance of my interface.
What I desire to have happen, is for OJB to properly cast each extent into 
it's appropriate subclass.
I've found that I can make this happen, but I'm worried that it might not 
be a safe method.

I have 2 tables, assets and computers.
Every computer is an asset, but not every asset is a computer.
I use the id field as the primary key for each table and as a foreign key 
for the computer table.
My repository structure is below.
I'm using the persistance broker interface to make calls with both 
getCollectionByQuery and getObjectByQuery;

What I've found is that OJB will generally cast all extents into a base 
"AssetBean" unless I add a redundant anonymous key to the ComputerAssetBean 
repository entry.

When there is 1 case of matching id fields in Assets and Computers and I 
call getObjectByQuery(Asset.class, id...) the results always come back cast 
as an AssetBean but I can see in the debug log that OJB at some point did 
try to look at the Computer table.
In the same circumstances, if I instead call 
getCollectionByQuery(Asset.class, id...) I will get back a collection with 
1 AssetBean, and 1 ComputerAssetBean, which is close enough to the desired 
behavior that I can work with it.

My question is:  Is this a safe practice?
Is there a better way to insure than an extent of an interface is properly 
cast into the most appropriate implimenting sub-class?




<class-descriptor class="org.aspca.vo.Asset">
    <extent-class class-ref="org.aspca.vo.AssetBean" />
    <extent-class class-ref="org.aspca.vo.ComputerAssetBean" />
  </class-descriptor>

<class-descriptor class="org.aspca.vo.AssetBean" table="INVENTR.ASSETS">
    <extent-class class-ref="org.aspca.vo.ComputerAssetBean" />
    <field-descriptor name="id" primarykey="true" nullable="false" 
default-fetch="true" column="ASSETID" jdbc-type="SMALLINT" autoincrement="true"
         access="readonly"/>
  </class-descriptor>

<class-descriptor class="org.aspca.vo.ComputerAssetBean" 
table="INVENTR.COMPUTERS">
    <field-descriptor name="id" primarykey="true" nullable="false" 
default-fetch="true" column="ASSETID" jdbc-type="SMALLINT" access="readonly"/>
    <field-descriptor name="assetId"  primarykey="true" nullable="false" 
default-fetch="true" column="ASSETID" jdbc-type="SMALLINT" access="anonymous"/>

    <field-descriptor name="RAM"  default-fetch="true" column="RAM" 
jdbc-type="INTEGER"/>
	 <field-descriptor name="computerName" default-fetch="true" 
column="COMPUTERNAME" jdbc-type="CHAR"/>
    <reference-descriptor name="super"
         class-ref="org.aspca.vo.AssetBean"
         auto-retrieve="true"
         auto-update="true"
         auto-delete="true"
     >
         <foreignkey field-ref="assetId"/>
   </reference-descriptor>
</class-descriptor>



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