["python","django","django-models","django-signals"]
Django post_save() signal implementation

I have a question about django. I have ManyToMany Models here class Product(models.Model): name = models.CharField(max_length=255) price = models.DecimalField(default=0.0, max_digits=9, decimal_places=2) stock = models.IntegerField(default=0) def __unicode__(self): return self.name class Cart(models.Model): customer = models.ForeignKey(Customer) products = models.ManyToManyField(Product, through='TransactionDetail') t_date = models.DateField(default=datetime.now()) t_sum = models.FloatField(default=0.0) def __unicode__(self): return str(self.id) class TransactionDetail(models.Model): product = models.ForeignKey(Product) cart = models.ForeignKey(Cart) amount = models.IntegerField(default=0) For 1 cart object created, I can insert as many as new TransactionDetail object (the product and amount). My question is. How can I implement the trigger? What I want is whenever a Transaction detail is created, I want the amount of the product's stock is substracted by the amount in the transactiondetail. I've read about post_save() but I'm not sure how to implement it. maybe something like this when: post_save(TransactionDetail, Cart) #Cart object where TransactionDetail.cart= Cart.id Cart.stock -= TransactionDetail.amount
If you really want to use signals to achieve this, here's briefly how, from django.db.models.signals import post_save from django.dispatch import receiver class TransactionDetail(models.Model): product = models.ForeignKey(Product) # method for updating @receiver(post_save, sender=TransactionDetail, dispatch_uid="update_stock_count") def update_stock(sender, instance, **kwargs): instance.product.stock -= instance.amount instance.product.save()