最近刚学习NHibernate，但是做一个多表查询就出了问题

 _session.CreateQuery("select distinct c from Usr c ,"
                + " c.OrderTbls o where o.OrderDesc like :orderDate").SetString("orderDate", "%" + strOrderDesc + "%")
                .List<Usr>();

上面是我写的HQL，但总是报c.OrderTbls is not mapped [select distinct c from myProject.Model.Usr c , c.OrderTbls o where o.OrderDesc like :orderDate]

我的XML是
   <id name="UserId" column="UserId" type="System.Int64" unsaved-value="0">
      <generator class="native" />
    </id>
    <property name="UserName" column="UserName" type="System.String" />
    <property name="Password" column="PassWord" type="System.String" length="50" />
    <property name="State" column="State" type="System.String" length="50" />
    <property name="CreateTime" column="CreateTime" type="System.DateTime" />

    <!--一对多关系：Customer有一个或多个Orders-->
    <set name="OrderTbls" table="OrderTbl" generic="true" inverse="true">
      <key column="UserId" foreign-key="FK_OrderTbl_Usr"/>
      <one-to-many class="myProject.Model myProject.Model.OrderTbl,"/>
    </set>

请高手帮忙指点一下 感激不尽！！！！！！