Use the getParentValue() method instead of aq_parent, which is
defined on Base class. aq_parent is only useful if you wish to get the
acquisition context of an object, which may be different from its physical
container. For example, aq_parent is useful to
calculate a breadcrumb whenever the acquisition path results from the combination
of multiple physical path (ex. web_site_module/a/b/web_page_module/c). Refer to
the Zope Developer Guide for more information on acquisition.