Thanks Evans.That was helpful. :) On Thursday, February 7, 2013 7:54:36 PM UTC+5:30, Tom Evans wrote: > > On Thu, Feb 7, 2013 at 1:54 PM, vijay shanker > <[email protected]<javascript:>> > wrote: > > 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 ? > > > > 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 >
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