Want do query Object width no 1-n reference

Hello List,

I want to have all "Users", which are not in a "Group"

Here my repository:

<class-descriptor class="de.on_ergy.lakon.data.model.User" table="user">
    <field-descriptor name="objId"       column="obj_id"         
jdbc-type="INTEGER"                primarykey="true" 
autoincrement="true"></field-descriptor>
    <field-descriptor name="username"   column="user_name"      
jdbc-type="VARCHAR"  length="100"   ></field-descriptor>
    <!-- m - n ÃŒber benutzer_gruppen zu gruppen -->
    <collection-descriptor
     name="groups"
     
collection-class="org.apache.ojb.broker.util.collections.ManageableArrayList"
     element-class-ref="de.on_ergy.lakon.data.model.Group"
     auto-retrieve="true"
     auto-update="false"
     auto-delete="link"
     proxy="true"
     indirection-table="user_group"
  >
     <fk-pointing-to-this-class column="user_obj_id"/>
     <fk-pointing-to-element-class column="group_obj_id"/>
  </collection-descriptor>   
</class-descriptor>

How can I get all Users, which currently not in a Group?
Is there a way like this?
Criteria crit = new Criteria();
crit.addIsNull("groups");

Thanks a lot for your help!

regards

-- 
Josef Wagner