hello....i'm newbie developer....

i really need help at now... i just get started with Nhibernate thing at .Net... when i learn Inheritance and try it...it makes me confusing...why i get error like this :

Initializing[AP.Core.Domain.AccountPayable.APInvoice#API03/04/2010/001]-Could not initialize proxy - no Session.

this is my xml :

  <class xmlns="urn:nhibernate-mapping-2.2" mutable="true" name="AP.Core.Domain.AccountPayable.APAdjustment, AP.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="APAdjustment">
    <id name="AdjustmentNumber" type="System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="AdjustmentNumber" length="17" />
      <generator class="assigned" />
    </id>
    <property name="Amount" type="System.Decimal, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Amount" />
    </property>
    <property name="TransactionDate" type="System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="TransactionDate" />
    </property>
    <many-to-one class="AP.Core.Domain.AccountPayable.APInvoice, AP.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" lazy="proxy" name="PurchaseInvoice">
      <column name="PurchaseInvoice_id" not-null="true" />
    </many-to-one>
    <joined-subclass name="AP.Core.Domain.AccountPayable.APCreditAdjustment, AP.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" lazy="true" table="APCreditAdjustment">
      <key>
        <column name="APAdjustment_id"  />
      </key>
    </joined-subclass>
  </class>
</hibernate-mapping>

and this is inheritance Class :

Parent Class -->

public class APAdjustment
        {
        #region :FIELD
        private string adjustmentNumber;
        private decimal amount;
        private DateTime transactionDate;
        private APInvoice purchaseInvoice;

Child Class -->

public class APCreditAdjustment : APAdjustment {

  public APCreditAdjustment(){

and this my Data access :

public IList<APAdjustment> GetByNameAll()
{
   ICriteria criteria = Nhibernatesession.CreateCriteria(typeof(APAdjustment));
   return criteria.List<APAdjustment>() ;
}

My Problem is :

1. when i load data with gridview ...it works...but i change the property to autogenerate="true" ...i missing "PurchaseInvoice" field...and i change to bind manually,and it works..when i edit that gridview ...i get this error...

   Initializing[AP.Core.Domain.AccountPayable.APInvoice#API03/04/2010/001]-Could not initialize proxy - no Session

2. so then i change my xml ...lazy="no-proxy" ...it still work...but when edit again ...i get error again ..and i do "Comment out the selected lines" to my association "Many-to-one"...i really works it..but that's not i want...

CAN ANYBODY HELP ME...??Plizz...:(

Note : I almost forget it ,i use fluent hibernate to generate to database.From fluent Hibernate ..i put *.xml file ...so i'm work to xml NHibernate...not fluent hibernate thing...:)