<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="BookToolV2" namespace="BookTool">
  <class name="ChapterHelper" table="CHAPTER" lazy="true" >
    <id name="ChapterId" column="ID_CHAPTER" type="Decimal">
      <generator class="assigned" />
    </id>
    <property name="BookId" type="Decimal" column="ID_BOOK" />
    <property name="ChapterType" type="Decimal" not-null="true" column="CHAPTER_TYPE"/>
  </class>
</hibernate-mapping>

class ChapterHelper
  {
    public virtual Decimal ChapterHelperId { get; set; }
    public virtual Decimal BookId { get; set; }
    public virtual Decimal ChapterType { get; set; }



}

class MyInterceptor: IInterceptor
{
...
public bool OnFlushDirty(object entity, object id, object[] currentState, object[] previousState, string[] propertyNames, NHibernate.Type.IType[] types)
    {
      if (entity.GetType().IsSubclassOf(typeof(Chapter)))
      {
        Chapter chapter = (Chapter)entity;
        Chapter checkChapter;
        ISession checkSession = NHibernateFactory.CheckSession; // a second session


        checkChapter = (Chapter)checkSession.Get(typeof(Chapter), (Object)chapter.Id);


        if (checkChapter == null)
        {
          DialogResult result = MessageBox.Show("Instance was not found in DB!");
          ChapterHelper helpchapter= new ChapterHelper();
          helpchapter.ChapterHelperId =chapter.Id;
          helpchapter.BookId = chapter.BookId;
          helpchapter.ChapterType = chapter.GetStypeTypeId(); // in the main mapping file this is a discriminator


          checkSession.Save(helpchapter);
          checkSession.Flush();
        }
      }


      return false;
    }
...
}