Generating tables with relationships in arc and part of primary key, bug?

I have three entites A, B and C. A relates to B and C with an arc and
(B,C)one-to-many(A). Both relationships are part of the UID of A.

Like this:
-- optional relationship
| Part of UID
( arc

< many

   B ----|-(-< A >-)-|--- C

B: #B_ID
C: #C_ID

1. #A_ID #B_ID
   (or) #C_ID
   

Hence my primary key on A should be

A_ID  NOT NULL
B_ID  NULL
C_ID  NULL

This is (probably) not possible for a primary key, but a unique key should solve it?

My actual problem relates to generation of tables from the entities. The Database Design Wizard gives table A with B_ID and C_ID as NULL and part of primary key. The Server Generator gives DDL accordingly but when the DDL is executed the status of B_ID and C_ID go from NULL to NOT NULL, hence making it impossible to enforce a mutually exclusive relationship.

My conclusion is that the Generator/Wizard does not realize the problem with arcs and primary keys.

If so, it's a bug! (IMHO, of course) The Wizard should have generated an unique key instead.

Would it help if the relationships were mandatory?

Any input appreciated!

oli

PS: Des2k 1.2

       oracle v7.1

--
Ole Christian Meldahl
"Sailor" 
Norwegian High Command, Information Systems
meldahl_at_pvv.unit.no