<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0"
    namespace="xxx" assembly="xxx">

    <class name="BuildingComponent" table="BuildingComponent" discriminator-value="0">

        <id name="Id" unsaved-value="-9999">
            <column name="BctId" sql-type="Int32" not-null="true"/>
            <generator class="increment" />
        </id>
          
        <discriminator column="BctType" type="int"/>
           
   <property name="Name" column="BctName" />
   Other properties ..
          
          <subclass name="PropertyBuildingComponent" discriminator-value="1">
      <many-to-one name="Property" class="Property" column="PrpId" />
   </subclass>
      
   <subclass name="FloorBuildingComponent" discriminator-value="2">
      <many-to-one name="Property" class="Property" column="PrpId" />
      <many-to-one name="Floor" class="Floor" column="FlrId" />
   </subclass>
      
   <subclass name="RoomBuildingComponent" discriminator-value="3">
      <many-to-one name="Property" class="Property" column="PrpId" />
      <many-to-one name="Floor" class="Floor" column="FlrId" />
      <many-to-one name="Room" class="Room" column="RomId" />
   </subclass>      
   
    </class>

</hibernate-mapping>