<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping auto-import="true"
                   package="com.tahonen.domain">
   <class name="User" table="usr">
      <id name="id" unsaved-value="0">
         <generator class="native"/>
      </id>
      <property name="username" column="usr_nam"/>
      <property name="password" column="pwd"/>
      <property name="email" column="email"/>
      <property name="phone" column="phone"/>
      <property name="location"  column="loc"/>
      <property name="birthYear" column="birth_year"/>
      <property name="description" type="text"/>
      <set name="groups" table="usr_group" lazy="true" cascade="all">
          <key>
              <column name="usr_id" not-null="true"/>
          </key>
          <many-to-many class="Grp">
              <column name="grp_id" not-null="true"/>
          </many-to-many>
      </set>
   </class>
   <class name="Grp" table="grp">
      <id name="id" unsaved-value="0">
         <generator class="native"/>
      </id>
      <property name="name"/>
      <property name="created"/>
      <many-to-one name="owner" class="User" column="owner"/>
   </class>
</hibernate-mapping>

. . .
Session s = HibernateUtil.currentSession();
User usr = new User();
usr.setUsername("username");
usr.setPassword("password");
usr.setCreated(new Date());
Set groups = new HashSet();
Grp g = ....//read grp from database
//I have tested that variable g is != null
groups.add(g);
usr.setGroups(groups);
s.save(usr);