Django snippets

def anti_loop(field_data, all_data):
    """
    Make sure we don't create an infinite loop with a
    self-referencing foreign key.
    
    Assumes a required, unique slug field ('slug') and an optional 
    self-referencing foreign key ('parent'). All_data doesn't give 
    you the object's ID, so we will find it via the unique slug.
    If either is not present we pass -- if there's no parent chosen 
    it's not a problem, and if there is no slug present the 
    submission will fail on that validation instead. It is worth 
    noting that if the user changes the slug field on submission AND 
    picks a bad parent it will not be caught.
    
    Infinite loop cases:
    1.    A references A
    2.    A tries to reference B, which is currently referencing A
    """
    
    parent = all_data.get('parent', '')
    slug = all_data.get('slug', '')
    error_msg = u"Please chose a parent that does not result in an infinite loop!"
    if parent and slug:
        try:
            this_category = Category.objects.get(slug = slug)
            parent_category = Category.objects.get(pk = int(parent))
            if parent_category.id == this_category.id or parent_category.parent == this_category:
                raise ValidationError(error_msg)                
        except ObjectDoesNotExist:
            pass