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To: jdo-user@db.apache.org
From: Drew Lethbridge <droo@bund.com.au>
Subject: column sharing
Date: Fri, 26 Oct 2007 12:31:45 +1000
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Greetings,

I have a question about JDO2.0.  When using an inheritance strategy  
of "superclass-table", is it valid for two separate subclasses which  
both have an identically-typed field to actually share a database  
column, rather than specifying a separate column for each of the  
fields?  As far as I can tell, the JDO spec is not clear on this.  Is  
it completely JDO-vendor implementation dependent (i.e. non-portable)?

Below is an illustrative example.  B extends A and also C extends A.   
The fields B.f1 and C.f2 are both declared as references to a  
persistence type "F" but defined to share the column named F_JDOID.   
Any potential problems with this?

Thanks.

Drew.

<class name="A" table="A">
     <!-- abstract superclass -->
     <datastore-identity column="JDOID" />
     <inheritance>
         <discriminator strategy="value-map" column="CLASSIND" />
     </inhertance>
     <version strategy="version-number" column="JDOVERSION" />
</class>

<class name="B">
     <!-- extends A -->
     <inheritance strategy="superclass-table" />
         <discriminator value="B" />
     </inheritance>
     <field name="f1" column="F_JDOID">
         <foreign-key deferred="true" />
     </field>
</class>

<class name="C">
     <!-- extends A -->
     <inheritance strategy="superclass-table">
         <discriminator value="C" />
     </inheritance>
     <field name="f2" column="F_JDOID">
         <foreign-key deferred="true" />
     </field>
</class>

<class name="F" table="F">
    <datastore-identity column="JDOID" />
    <!-- etc... -->