<class name="Actor" abstract="true">
       
        <id name="identifier" type="long">
            <generator class="hilo" />
        </id>
        <version name="entityRevision" />
       
        <union-subclass name="Group" table="UserGroup">
            <property name="name" type="string" unique="true"/>
            <many-to-one name="parentGroup" />
            <set name="childrenGroups" inverse="true">
                <key property-ref="parentGroup" />
                <one-to-many class="Group" />
            </set>
        </union-subclass>

        <union-subclass name="User">
            <property name="disabled" type="boolean" />
            <property name="eMail" type="string"/>
            <property name="accountName" type="string" />
            <property name="fullName" type="string" />
            <property name="encPassword" type="string" />
        </union-subclass>
    </class>

3766 ERROR org.hibernate.tool.hbm2ddl.SchemaExport
  Unsuccessful: alter table UserGroup add constraint FK8A5BE15445D99518 foreign key (parentGroup) references UserGroup
3776 ERROR org.hibernate.tool.hbm2ddl.SchemaExport
  Constraint not foundtable has no primary key in statement [alter table UserGroup add constraint FK8A5BE15445D99518 foreign key (parentGroup) references UserGroup]