Django cannot make m2m_changed change the same m2m field

Question

My models looks something like this:

class Store:
    category = ManyToManyField(Category)
class Category:
    parent = ForeignKey(Category)

This is what I want to do:

def update_storem2m_handler(instance, action, reverse=False, pk_set=[], **kwargs):
    for c in instance.category.all():
        instance.category.add(c.parent)
m2m_changed.connect(update_storem2m_handler, sender=Store.category.through)

But alas it doesn't work. My current solution is the following:

update_storem2m_handler_lock = False #ugly hack to make a lock. NONCONCURRENT!
def update_storem2m_handler(instance,
        action="post_add", reverse=False, pk_set=[], **kwargs):
    """If a category is connected, it's parent should be to"""
    #should we run this function? probably not
    global update_storem2m_handler_lock
    if update_storem2m_handler_lock:
        return
    if reverse:
        for p in pk_set:
            update_storem2m_handler(Store.objects.get(pk=p),
                    action, reverse, [instance.pk], **kwargs)
        return
    if not action in ["post_add"]: #, "post_remove", "post_clear"]:
        return

    #yes we can run this function
    ps=[]

    for c in instance.category.all():
        ps.append(c)

    #remove everything
    instance.category.clear()
    update_storem2m_handler_lock = True
    #and under lock, add everything we want
    instance.category.add(*ps)
    update_storem2m_handler_lock = False

I have a problem with it though: it is really ugly!!! Anyone have any idea on how to make it nice (and efficient)?