<hibernate-mapping default-cascade="none" xmlns="urn:nhibernate-mapping-2.2">
  <class name="Header" table="Header" lazy="true">

    <id name="HeaderId" type="System.Int32" column="HeaderId" unsaved-value="0">
      <generator class="native" />
    </id>
    <property name="Name" type="System.String" column="Name" not-null="true" length="50" />

  </class>
</hibernate-mapping>

<hibernate-mapping default-cascade="none" xmlns="urn:nhibernate-mapping-2.2">
  <class name="HeaderDetail" table="HeaderDetail" lazy="true">

    <composite-id name="HeaderDetailPrimaryKey" class="PK_HeaderDetail" unsaved-value="any" access="property">
      <key-property name="ColumnId" type="System.Int32" column="ColumnId" />
      <key-many-to-one name="HeaderObj" class="Header">
        <column name="HeaderId" />
      </key-many-to-one>
    </composite-id>
    <property name="Desc" type="System.String" column="Desc" not-null="false" >
    <property name="Qty" type="System.Int32" column="Qty" not-null="false" />

  </class>
</hibernate-mapping>

   [System.SerializableAttribute()]
    public class PK_HeaderDetail
    {
        private int columnId;

        private Header headerObj;

        public virtual int ColumnId
        {
            get
            {
                return this.columnId;
            }
            set
            {
                this.columnId= value;
            }
        }

        public virtual Header HeaderObj        {
            get
            {
                return this.headerObj;
            }
            set
            {
                this.headerObj= value;
            }
        }

        public override int GetHashCode()
        {
            return base.GetHashCode();
        }

        public override bool Equals(object obj)
        {
            if (((obj == null)
                        || (obj == System.DBNull.Value)))
            {
                return false;
            }
            PK_HeaderDetail entity = ((PK_HeaderDetail)(obj));
            return ((ColumnId== entity.ColumnId)
                        && (HeaderObj == entity.HeaderObj));
        }
    }

public class Header {
   
   private long headerId;
   
   private List detailList;

   /**
    *
    */
   public Header() {
      super();
   }
   /**
    * @return Returns the headerId.
    */
   public long getHeaderId() {
      return headerId;
   }
   /**
    * @param headerId The headerId to set.
    */
   public void setHeaderIdId(long headerId) {
      this.headerId = headerId;
   }

   /**
    * @return Returns the details.
    */
   public List getDetailList() {
      return detailList;
   }
   /**
    * @param details The detail to set.
    */
   public void setDetailList(List details) {
      this.detailList = details;
   }
}

ISession sess=NHSession.Instance;
ICriteria crit=sess.CreateCriteria(typeof(HeaderDetail));

crit.Add(Expresion.Eq("HeaderDetailPrimaryKey.HeaderObj",myHeaderObject));

IList detailsForMyHeader=crit.List<HeaderDetail>();