Hibernate mapping problem of parent

The mapping attribute parent="xxx" doesn't work

Hibernate Version: version 3.2.5

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping
DTD//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="parenttest">
   <class name="Parent" table="PARENT">
      <id name="id" column="PARENT_ID" type="long">
         <generator class="native"/>
      </id>
      <property name="name" column="NAME" type="string"/>

      <set name="Childs" table="CHILDS">
         <key column="CHILD_ID"/>
         <composite-element class="Child">
            <parent name="Parent"/>
            <property name="username" column="ADDED_BY_USER"
type="string"/>
         </composite-element>
      </set>
   </class>
</hibernate-mapping>

But I get the Child.java class source file by hbm2java.

package parenttest;
// Generated 2007-9-9 17:42:40 by Hibernate Tools 3.2.0.b9

/**
 * Child generated by hbm2java
 */
public class Child implements java.io.Serializable {

     private String username;

    public Child() {
    }

    public Child(String username) {
       this.username = username;
    }

    public String getUsername() {
        return this.username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

The code of Parent.java is OK.

So what's wrong with the mapping files? I want the Child to have the
getter and setter of parent. I see the parent tag do this work in
hibernate document.

   private Parent parent;
   public Parent getParent() {
      return parent;
   }

   public void setParent(Parent parent) {
      this.parent = parent;
   }