nhibernate Polymorphic association

Question

I have a problem width Polymorphic association in nhibernate.

DocumentLocation Mapping :

<class name="Document.DocumentLocation" table="DocumentLocation">
    <id name="DocumentLocationId" type="Int32" unsaved-value="null">
     <column name="DocumentLocationId" length="4" sql-type="int" not-null="true" unique="true" index="PK_DocumentLocation"/>
     <generator class="increment" />
    </id>
    <many-to-one name="Parent" class="DocumentLocation">
     <column name="ParentDocumentLocationId" length="4" sql-type="int" not-null="false"/>
    </many-to-one>
    <many-to-one name="Document" class="Document" not-null="false">
        <column name="DocumentId" length="4" sql-type="int" not-null="true"/>
    </many-to-one>
    <joined-subclass name="Receiver" schema="document" lazy="true" table="Receiver">
      <key>
        <column name="DocumentLocationId" length="4" sql-type="int" not-null="true" unique="true"/>
      </key>
      <one-to-one name="Refer" property-ref="Receiver" cascade="all-delete-orphan" class="Refer" />
    </joined-subclass>
    <joined-subclass name="Sender" schema="document" lazy="true" table="Sender">
      <key>
        <column name="DocumentLocationId" length="4" sql-type="int" not-null="true" unique="true"/>
      </key>
    </joined-subclass>
  </class>

There is an one-to-many association in document class

 <bag name="Receivers" inverse="true" lazy="true" cascade="all-delete-orphan"  >
     <key column="DocumentId"/>
     <one-to-many class="DocumentLocation"/>
 </bag>

But Receivers contain both Sender and Receiver. Is there anyway to tell nhibernate select only one subclass in polymorphic association?