Django post_save() signal implementation

Question

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 

Answer 1

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() 

Answer 2

Personally I would override the TransactionDetail's save() method and in there save the new TransactionDetail and then run

self.product.stock -= self.amount self.product.save()