class Customer : Company

class Customer { ... public CustomerInternetServiceDetail CustomerInternetServiceDetail { get; set; }... }

(Customer)companyManager.FindCompanyByID( companyID );

   <class name="Company" table="Company">
      <id name="CompanyID" column="CompanyID" type="Int32" unsaved-value="0">
         <generator class="identity" />
      </id>
      <property unique="true" column="CompanyName" type="String" name="CompanyName" not-null="true" length="250" />
      <property column="CompanyTypeID" name="CompanyTypeID" not-null="true" />
      <set name="Contacts" inverse="true" cascade="all" table="Contacts">
         <!--jcs-cache usage="read-write" /-->
         <key column="CompanyId" />
         <one-to-many class="Contact" />
      </set>
      <set name="Addresses" inverse="true" cascade="all" table="Address">
         <!--jcs-cache usage="read-write" /-->
         <key column="CompanyId" />
         <one-to-many class="Address" />
      </set>
   </class>

   <joined-subclass name="Customer" table="Customer" extends="Company" >

      <key column="CustomerID" />

      <!-- other stuff here -->
      <one-to-one name="InternetServiceDetail" class="CustomerInternetServiceDetail" cascade="all-delete-orphan" fetch="join" outer-join="true" />
   </joined-subclass>

   <class name="CustomerInternetServiceDetail" table="CustomerInternetServiceDetail">

      <id name="CustomerInternetServiceDetailID" column="CustomerID" type="Int32">
         <generator class="foreign" >
            <param name="property">Customer</param>
         </generator>
      </id>
      <one-to-one name="Customer" class="Customer" constrained="true" cascade="none" />
      <!-- other stuff here -->
      
   </class>

_session.Get( typeof(Customer), customerID)

      public void SaveCompany(Company company, UserAccount modifiedBy)
      {
         if( company == null )
            throw new ArgumentNullException("company","Company required.");

         if( modifiedBy == null )
            throw new ArgumentNullException("modifiedBy","UserAccount required.");

         // open another session on the same connection (we have to close it if the transaction fails)
         using(ISession session = NHbServices.SessionFactory.Instance.NHibernateFactory
                 .OpenSession( _session.Connection, new NHbServices.AuditInterceptor( modifiedBy.UserAccountID )))
         {
            using(ITransaction tr = session.BeginTransaction())
            {
               try
               {
                  _session.Evict( company );
                  session.SaveOrUpdate( company );
                  tr.Commit();

                  session.Close();
               }
               catch( Exception e )
               {
                  tr.Rollback();
                  throw new ApplicationException("Unable to save the company.", e );
               }
            }
            _session.Refresh( company ); // reassociate with the session that stays open (in case there's anything lazy-loading later
         }
      }