public class Operation
{
   private int id;
   private PaymentPlan paymentPlan;
       
       //getter and setter
}

public class PaymentPlan
{
   private DateTime expirationDate;
   private int numberOfPayments;
   private IList payments;
       
        //getter and setter
}

public class Payment
{
   private int id;
   private int amount;
   private DateTime expiration;
   private PaymentPlan paymentPlan;

        //getter and setter
}

<?xml version="1.0" encoding="utf-16"?>
<hibernate-mapping xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="urn:nhibernate-mapping-2.0">
  <class name="Castle.ActiveRecord.Tests.Model.Nested.Operation, Castle.ActiveRecord.Tests" table="Operation" lazy="false">
    <id name="Id" access="property" column="Id" type="Int32" unsaved-value="0">
      <generator class="native">
      </generator>
    </id>
    <component name="PaymentPlan">
      <property name="ExpirationDate" access="property" column="ExpirationDate" type="System.DateTime" />
      <property name="NumberOfPayments" access="property" column="NumberOfPayments" type="Int32" />
      <bag name="Payments" access="property" table="Payment" lazy="false">
        <key column="PaymentPlan" />
        <one-to-many class="Castle.ActiveRecord.Tests.Model.Nested.Payment, Castle.ActiveRecord.Tests" />
      </bag>
    </component>
  </class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-16"?>
<hibernate-mapping xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="urn:nhibernate-mapping-2.0">
  <class name="Castle.ActiveRecord.Tests.Model.Nested.Payment, Castle.ActiveRecord.Tests" table="Payment" lazy="false">
    <id name="Id" access="property" column="Id" type="Int32" unsaved-value="0">
      <generator class="native">
      </generator>
    </id>
    <property name="Amount" access="property" column="Amount" type="Int32" />
    <property name="Expiration" access="property" column="Expiration" type="System.DateTime" />
    <many-to-one name="PaymentPlan" access="property" class="Castle.ActiveRecord.Tests.Model.Nested.PaymentPlan, Castle.ActiveRecord.Tests" column="PaymentPlan" />
  </class>
</hibernate-mapping>

TestCase 'Castle.ActiveRecord.Tests.JoinedSubClassWithNestedClassTestCase.Operations' failed: System.NullReferenceException : Object reference not set to an instance of an object.
   at NHibernate.Cfg.Mapping.GetIdentifierType(Type persistentClass)
   at NHibernate.Type.ManyToOneType.GetReferencedType(IMapping mapping)
   at NHibernate.Type.ManyToOneType.GetColumnSpan(IMapping mapping)
   at NHibernate.Mapping.SimpleValue.IsValid(IMapping mapping)
   at NHibernate.Mapping.Property.IsValid(IMapping mapping)
   at NHibernate.Mapping.PersistentClass.Validate(IMapping mapping)
   at NHibernate.Mapping.RootClass.Validate(IMapping mapping)
   at NHibernate.Cfg.Configuration.Validate()
   at NHibernate.Cfg.Configuration.BuildSessionFactory()
   D:\Progetti\Castle\ActiveRecord\Castle.ActiveRecord\Framework\SessionFactoryHolder.cs(115,0): at Castle.ActiveRecord.Framework.SessionFactoryHolder.GetSessionFactory(Type type)
   D:\Progetti\Castle\ActiveRecord\Castle.ActiveRecord\Framework\SessionFactoryHolder.cs(140,0): at Castle.ActiveRecord.Framework.SessionFactoryHolder.CreateSession(Type type)
   d:\progetti\castle\activerecord\castle.activerecord\framework\activerecordbase.cs(178,0): at Castle.ActiveRecord.ActiveRecordBase.DeleteAll(Type type)
   d:\progetti\castle\activerecord\castle.activerecord.tests\model\nested\operation.cs(31,0): at Castle.ActiveRecord.Tests.Model.Nested.Operation.DeleteAll()
   d:\progetti\castle\activerecord\castle.activerecord.tests\nestedclasswithbelongstotestcase.cs(24,0): at Castle.ActiveRecord.Tests.JoinedSubClassWithNestedClassTestCase.Operations()

<many-to-one name="PaymentPlan" access="property" class="Castle.ActiveRecord.Tests.Model.Nested.PaymentPlan, Castle.ActiveRecord.Tests" column="PaymentPlan" />

PersistentClass referencedClass = ( PersistentClass ) classes[ fk.ReferencedClass ];