I guess its better to have your object (the parent one like assignment) in parent class(submission) instead of subclass and if so then if you have assignment inside submission in order to create hibernate of submission you just need to define your object. but in general if you need to map your subclass inside your hibernate you can use 2 formats :

 <joined-subclass name="xxx" table="xxx">

              <key column="xxx_id"/>

             **** any attribute that you have inside your subclass here****


    </joined-subclass>

or 

 <subclass name="xxx" discriminator-value="">

**** any attribute that you have inside your subclass here****
           
                   
       
        </subclass>

look at the hibenate on answer.hbm.xml under hbm.survey to get some idea