'Override save to set Subscribers and send Notifications'

def old_save(self, *args, **kwargs):

    original = None
    original_assigned = []
    if hasattr(self, 'instance'):
        try:
            original = Task.objects.get(pk=self.instance.id)
            original_assigned = list(original.assigned.all())
        except Task.DoesNotExist:
            pass
    instance = super(TaskForm, self).save(*args, **kwargs)
    if original:
        new_assigned = list(self.cleaned_data['assigned'])
        if (original_assigned != new_assigned):
            for assignee in new_assigned:
                self.instance.subscribers.add(assignee)
    return instance
