post_save signal getting called twice !

[vijay shanker]

Hi
I am using django version 1.4.3
I am using two signals in my models.py
one is m2m_changed, so i can have a counter field (numcounter) of number of m2m fields attached, and another is post_save so i can decide whether to have a OneToOneField (cartrule, is to be applied only if there are more than 2 fields) or not.
my models.py is:

class CartItem(models.Model):
    content_type    = models.ForeignKey(ContentType)
    object_id       = models.PositiveIntegerField()
    content_object  = generic.GenericForeignKey('

content_type','object_id')
    quantity        = models.PositiveIntegerField(default=0)
    is_abandoned    = models.BooleanField(default=False)
    created_at      = models.DateTimeField(auto_now_add=True)
    update_at       = models.DateTimeField(auto_now=True)
    def __str__(self):
        return self.content_object.name

class CartRule(models.Model):
    ##some field
    pass
   
class Cart(models.Model):
    cart_id             = models.CharField(max_length=50, null=False)
    customer            = models.ForeignKey(Customer,null=True,blank=True)
    cartitems           = models.ManyToManyField(CartItem,null=True)
    created_at          = models.DateTimeField(auto_now_add=True)
    update_at           = models.DateTimeField(auto_now=True)
    cartrule               = models.OneToOneField(crapclass,null=True,blank=True)
    num_cartitem        = models.IntegerField()
    def __str__(self):
        return self.cart_id
 
@receiver(post_save, sender=Cart)
def apply_condition(sender,instance,created,raw,using,*args,**kwargs):
    # i want to decide here if num_cartitem is greater than 2 its ok to have a cartrule
    pass

@receiver(m2m_changed)
def save_cartitem_counter(sender, instance, signal,*args, **kwargs):
    if kwargs['action'] == 'post_add':
        instance.num_cartitem = instance.cartitems.all().count()
        instance.save()

the issue is apply_condition gets called twice, with similar value of args, first with older value of m2m (cartitem) field in Cart, the other time with the values i intended to save
I looked into older post but still could not figure out the whys.How should i go about this ?

[Tom Evans]

When you save a Cart instance, the post_save signal is triggered.
If the M2M relationship is changed as well, then the m2m_changed
signal is triggered. Your handler for this then re-saves the Cart
instance after denormalising data, which triggers the post save signal
for a second time.
You should probably connect the M2M receiver to a specific sender too,
currently it will fire whenever any M2M on any model is changed.

Keeping denormalized data like that is a pain, could you do without
it, and re-calculate it where necessary?

Alternatively, you could use a named intermediary M2M relationship:

https://docs.djangoproject.com/en/1.4/topics/db/models/#intermediary-manytomany

and connect signals on post_create and post_delete to the intermediary
model, updating the Cart as necessary.

This is a little more logical, since you wish to denormalise the data
whenever an item is added to or removed from a cart, ie whenever a row
is added or deleted to the intermediate table. Naming the relationship
allows you to connect the signals to the right place.

Cheers

Tom

[vijay shanker]

Thanks Evans.That was helpful. :)

[vijay shanker]

further, i tried putting a sender argument in receiver, but the sender , i found (by set_trace) is Cart_cartitems, how can i import this <class 'shoppingcart.models.Cart_cartitems'> when its only a intermediary model that django's creating.

[Martin J. Laubach]

further, i tried putting a sender argument in receiver, but the sender , i found (by set_trace) is Cart_cartitems, how can i import this <class 'shoppingcart.models.Cart_cartitems'> when its only a intermediary model that django's creating.

  I think you want Cart.cartitems.through

          mjl

[vijay shanker]

yes that was it. thanks a ton. ;)