<class name="com.domain.data.Head" table="head">
   <id name="headId" type="long" column="head_id">
      <generator class="native" />
   </id>

   <property    name="randomProperty" not-null="true"  type="long"      column="random_property_id"/>

   <map name="foobarMap" cascade="all">
      <key column="head_id" not-null="true"/>
      <map-key column="name" type="string"/>
      <one-to-many class="com.domain.data.FoobarMap"/>
   </map>
</class>


<class name="com.domain.data.FoobarMap" table="foobar_map">
   <id name="foobarMapId" type="long" column="foobar_map_id">
      <generator class="native" />
   </id>

   <many-to-one name="head" column="head_id" not-null="true" insert="false" update="false"/>
   <!-- Map key -->
   <property    name="name"               not-null="true"  type="string"    column="name" insert="false" update="false"/>
   <property    name="otherDataOne"       not-null="false" type="integer"   column="other_data_one"/>
   <property    name="description"        not-null="true"  type="string"    column="description"/>
</class>

FoobarMap foobarMap = new FoobarMap();
foobarMap.setName("the_name");
foobarMap.setDescription("hello");
foobarMap.setOtherDataOne(Integer.valueOf(1));

Head head = (Head) hsession.load(Head.class, new Long(1));
head.getFoobarMap.put(foobarMap.getName(), foobarMap);
hsession.save(foobarMap);

Head head = (Head) hsession.load(Head.class, new Long(1));
Iterator it = head.getFoobarMap().keySet().iterator();
while(it.hasNext()) {
   String thisName = (String) it.next();
   FoobarMap foobarMap = (FoobarMap) head.getFoobarMap().get(thisName);

   boolean remove = true;
   if(externalTest(head, foobarMap))
      remove = false;

   if(remove) {
      it.remove();
      // This line throws and exception org.hibernate.NonUniqueObjectException
      //  a different object with the same identifier value was already associated with the session: [com.domain.data.FoobarMap#0]
      // hsession.delete(foobarMap);
   }
}

// Sometimes I optionally add new entries into the head.getFoobarMap().put() here
//  those additions work correctly.  But because the delete part above is not working
//  they accumulate rows.

// I am hoping this will cascade delete, but it does not.
hsession.update(head);