6.3.3. Bidirectional associations with indexed collections

A bidirectional association where one end is represented as a <list> or <map> requires special consideration. If there is a property of the child class which maps to the index column, no problem, we can continue using inverse="true" on the collection mapping:
<class name="Parent">
    <id name="id" column="parent_id"/>
    ....
    <map name="children" inverse="true">
        <key column="parent_id"/>
        <map-key column="name" 
            type="string"/>
        <one-to-many class="Child"/>
    </map>
</class>

<class name="Child">
    <id name="id" column="child_id"/>
    ....
    <property name="name" 
        not-null="true"/>
    <many-to-one name="parent" 
        class="Parent" 
        column="parent_id"
        not-null="true"/>
</class>
But, if there is no such property on the child class, we can't think of the association as truly bidirectional (there is information available at one end of the association that is not available at the other end). In this case, we can't map the collection inverse="true". Instead, we could use the following mapping:
<class name="Parent">
    <id name="id" column="parent_id"/>
    ....
    <map name="children">
        <key column="parent_id"
            not-null="true"/>
        <map-key column="name" 
            type="string"/>
        <one-to-many class="Child"/>
    </map>
</class>

<class name="Child">
    <id name="id" column="child_id"/>
    ....
    <many-to-one name="parent" 
        class="Parent" 
        column="parent_id"
        insert="false"
        update="false"
        not-null="true"/>
</class>
Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key. TODO: Does this really result in some unnecessary update statements?