Thanks for your responses. I finally figured out what the problem was. The culprit was due to a  one-to-one mapping  on a primary key association.  I am not able to get this one to one association to work, even after following the text book definition of defining the primary key on the child.
Code:
<id name="CaseID" column="CaseID">
   <generator class="foreign"> 
   <param name="property">Case</param>
   </generator>            
</id>
Both sides of the relationship have been defined using one-to-one association with the constrained="true" set on the child side of the association.