Question

suppose Object A has a list of Object B, and Object B must have a object C, B can be order base on C.level .

In A.hbm.xml

<bag name="listB"
             table="T_B"
             inverse="false"
             order-by="?? what should i do here???"
         >
            <key column="ID_A" not-null="true"/>
            <many-to-many column="ID_B" class="B"/>
        </bag>

Answer 1

The order-by clause should contain the SQL snippet you would use to order the list of items. If your ordering criteria is not located in the table T_B in your example, then you probably won’t be able to accomplish what you want. On the other hand if B contains a C and its always a one to one relationship, you could define a view in the DB the aggregates the tables and gives you a column to sort on.