Id | Discr  | Name              | SpouseID
-------------------------------------------
1  |   M    | Robert            | NULL
2  |   W    | Laura             | NULL
3  |   MM   | Thomas            |  4
4  |   MW   | Anna              |  3

<class name="person" table="people">
    <subclass name="man" discriminator-value="M">
        <subclass name="marriedman" discriminator-value="MM">
            <many-to-one name="wife" column="spouseID">
        </subclass>
    </subclass>

    <subclass name="woman" discriminator-value="W">
        <subclass name="marriedwoman" discriminator-value="MW">
            <many-to-one name="husband" column="spouseID">
        </subclass>
    </subclass>
</class>

    public abstract class Person
    {
        private int _id;

        public int Id
        {
            get { return _id; }
            set { _id = value; }
        }
   
        private string _name;
        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
   
    }
    public class Man : Person
    {
    }
    public class Woman : Person
    {
    }
    public class MarriedMan : Man
    {
        private MarriedWoman _wife;

        public MarriedWoman Wife
        {
            get { return _wife; }
            set { _wife = value; }
        }
   
    }
    public class MarriedWoman : Woman
    {
        private MarriedMan _husband;

        public MarriedMan Husband
        {
            get { return _husband; }
            set { _husband = value; }
        }
    }

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" default-access="nosetter.camelcase-underscore" >
  <class name="SancorSeguros.Inversiones.Person, SancorSeguros.Inversiones" table="Person" discriminator-value="-1">
    <id name="Id" unsaved-value="0" access="nosetter.camelcase-underscore" >
      <generator class="identity" />
    </id>
    <discriminator column="type" />
    <property name="Name" />
    <subclass name="SancorSeguros.Inversiones.Man, SancorSeguros.Inversiones" discriminator-value="M">
      <subclass name="SancorSeguros.Inversiones.MarriedMan, SancorSeguros.Inversiones" discriminator-value="MM">
        <many-to-one name="Wife" column="spouseID" />
      </subclass>
    </subclass>
    <subclass name="SancorSeguros.Inversiones.Woman, SancorSeguros.Inversiones" discriminator-value="W">
      <subclass name="SancorSeguros.Inversiones.MarriedWoman, SancorSeguros.Inversiones" discriminator-value="WW">
        <many-to-one name="Husband" column="spouseID" />
      </subclass>
    </subclass>
  </class>
</hibernate-mapping>

Person person = (Person)sess.Load(typeof(Person), 3);

2005-12-27 15:29:03,078 [5808] INFO  NHibernate.Loader.Loader [(null)] &lt;(null)&gt; - SELECT person0_.Id as Id3_, person0_.type as type3_, person0_.Name as Name3_, person0_.spouseID as spouseID3_, marriedwom1_.Id as Id0_, marriedwom1_.spouseID as spouseID0_, marriedwom1_.Name as Name0_, marriedman2_.Id as Id1_, marriedman2_.spouseID as spouseID1_, marriedman2_.Name as Name1_, marriedman3_.Id as Id2_, marriedman3_.spouseID as spouseID2_, marriedman3_.Name as Name2_ FROM Person person0_ left outer join Person marriedwom1_ on person0_.spouseID=marriedwom1_.Id left outer join Person marriedman2_ on marriedwom1_.spouseID=marriedman2_.Id left outer join Person marriedman3_ on person0_.spouseID=marriedman3_.Id WHERE person0_.Id=@p0
2005-12-27 15:29:03,234 [5808] WARN  NHibernate.Util.ADOExceptionReporter [(null)] &lt;(null)&gt; - NHibernate.WrongClassException: Object with id: 4 was not of the specified sublcass: SancorSeguros.Inversiones.MarriedMan (loading object was of wrong class)
   at NHibernate.Loader.Loader.InstanceAlreadyLoaded(IDataReader rs, Int32 i, ILoadable persister, String suffix, Key key, Object obj, LockMode lockMode, ISessionImplementor session)
   at NHibernate.Loader.Loader.GetRow(IDataReader rs, ILoadable[] persisters, String[] suffixes, Key[] keys, Object optionalObject, Key optionalObjectKey, LockMode[] lockModes, IList hydratedObjects, ISessionImplementor session)
   at NHibernate.Loader.Loader.GetRowFromResultSet(IDataReader resultSet, ISessionImplementor session, QueryParameters queryParameters, IList hydratedObjects, Object optionalObject, Object optionalId, Key[] keys, Boolean returnProxies)
   at NHibernate.Loader.Loader.DoQuery(ISessionImplementor session, QueryParameters queryParameters, Object optionalObject, Object optionalId, Object[] optionalCollectionKeys, Boolean returnProxies)
2005-12-27 15:29:03,250 [5808] ERROR NHibernate.Util.ADOExceptionReporter [(null)] &lt;(null)&gt; - Object with id: 4 was not of the specified sublcass: SancorSeguros.Inversiones.MarriedMan (loading object was of wrong class)

Person person = (Person)sess.Load(typeof(MarriedMan), 3);

Person person = (Person)sess.Load(typeof(Person), 4);

Person person = (Person)sess.Load(typeof(Person), 3);

FROM Person p WHERE p.id = :Id

Person person = (Person)sess.Load(typeof(Person), 3); 

<class name="person" table="people">
    <subclass name="man" discriminator-value="M">
        <subclass name="marriedman" discriminator-value="MM">
            <many-to-one name="wife" class="marriedwoman" column="spouseID">
        </subclass>
    </subclass>

    <subclass name="woman" discriminator-value="W">
        <subclass name="marriedwoman" discriminator-value="MW">
            <many-to-one name="husband" class="marriedman" column="spouseID">
        </subclass>
    </subclass>
</class>