Why can't one extend union-subclass with subclass ?

ogeisser · **Joined:** Wed Aug 18, 2004 2:31 pm **Posts:** 1

Hibernate version 3.0 RC

After reading the Reference manual and the H3 DTD I do not quite understand the following limitation in Hibernate 3:

The DTD does not allow to extend a union-subclass with a subclass element.

What we want to achieve is to map the following class hierarchy:

Code:

              (abstract)
                 Top
                /   \
               /     \
    (abstract)        (abstract)
     Middle1            Midlle2
    /       \          /       \
 Base1a   Base1b    Base2a   Base2b

The Middle1 and Middle2 hierarchies should both be mapped with table-per-class-hierarchy mappings, e.g. one table for Middle1 and it's subclasses and one table for Middle2 and it's subclasses.

So we thought this mapping should work:

Code:

<class name="Top" abstract="true">
  ...

  <union-subclass name="Middle1" abstract="true">
    <subclass name="Base1a">
       ...

    <subclass name="Base1b">
       ...

  <union-subclass name="Middle2" abstract="true">
    <subclass name="Base2a">
       ...

    <subclass name="Base2b">
       ....

As said above the DTD does not allow this. So we want to know if there is another way to express our mapping?
Or is this limitation a bug in the DTD?

Thanks, Olli