'Return scrubbed subproject choice queryset
This removes projects that are either already a subproject of another
project, or are a superproject, as neither case is supported.'

def get_subproject_queryset(self):

    queryset = Project.objects.for_admin_user(self.user).exclude(subprojects__isnull=False).exclude(superprojects__isnull=False)
    return queryset
