<class name="${user.pkg}.UserToGroup"
many-to-many="true"
factory-ext="ManyToManyFactoryBase"
>
<map-to table="merlin_user_desk"/>
<field name="user" type="${user.pkg}.User"
>
<sql name="user_oid"/>
</field>
<field name="desk" type="${user.pkg}.Group">
<sql name="desk_oid"/>
</field>
</class>
Note: the field many-to-many was set to true : and the base factory was set to ManyToManyFactoryBase.