Q:

polymorphic hql

I have a base type where "business id" must be unique for a given subclass, but it is possible for there to be different subclasses with the same business id. 
If there is a base type with a requested id but of the wrong subclass I want to return null, using a named query. The code below does this, but I am wondering if I can avoid the try/catch with a better HQL. Can I?
Cheers,
Berryl
current hql
<query name="FindActivitySubjectByBusinessId">
<![CDATA[
          from ActivitySubject act 
          where act.BusinessId = :businessId 
]]>

</query>

current fetch code
    public ActivitySubject FindByBusinessId<T>(string businessId) where T : ActivitySubject
    {
        Check.RequireStringValue(businessId, "businessId");
        try {
            return _session.GetNamedQuery("FindActivitySubjectByBusinessId")
                .SetString("businessId", businessId)
                .UniqueResult<T>();
        }
        catch (InvalidCastException e) {
            // an Activity Subject was found with the requested id but the wrong type
            return null;
        }
    }

A:

Query by discriminator in NHibernate

