In Grails, setting tablePerHierarchy false doesn't create a foreign key relationship

Question

We have our base class as something like package com.ids.emr.diag

class Base {

String foo static mapping = { tablePerHierarchy false } }

and the subclass as

class Sub extends Base { String bar }

This does not create a foreign key relationship between the base class and the subclass. Is there anything wrong that we're doing here?