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;
  }