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

You're likely to run into race conditions if you do it that way. — Thomas Orozco
– Thomas Orozco, Commented Oct 22, 2012 at 17:29

Ben J. · Accepted Answer · 2019-11-14 23:21:19Z

212

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()

edited Nov 14, 2019 at 23:21

Ben J.

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answered Oct 22, 2012 at 17:56

Kenny Shen

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9 Comments

Manoj Sahu Over a year ago

THis is working fine for me but do not know why it is in loop of unknown length

Dipak Over a year ago

I am getting maximum recursion depth exceeded error, because I was saving the instance itself in the @receiver function. How can I achieve to update self models ? Do I have to override save() method of models?

Vamsidhar Muggulla Over a year ago

@Dipak maximum recursion depth exceeded because every time you update the instance the post_save is triggered so it is calling itself on every save, at a state the recursion depth value exceeds the exception arises. let me know how did you overcome that

Dipak Over a year ago

@VamsidharMuggulla Instead of using signal I override save method of model and using updated function updated model property, so that it wont trigger save again.

R11G Over a year ago

What is dispatch_uid for?

|

Mikael · Accepted Answer · 2012-10-22 17:14:13Z

23

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()

answered Oct 22, 2012 at 17:14

Mikael

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2 Comments

blong Over a year ago

Does this work and does it appropriately handle the database transaction / rollback?

Mikael Over a year ago

Yes if the entire transaction is run in atomic mode

Guglielmo Celata · Accepted Answer · 2017-12-28 12:24:03Z

22

If you want to avoid getting maximum recursion depth exceeded, then you should disconnect signals, before saving within the signal handler. The example above (Kenny Shen's answer), would then be:

from django.db.models.signals import post_save
from django.dispatch import receiver

class TransactionDetail(models.Model):
    # ... fields here

# method for updating
@receiver(post_save, sender=TransactionDetail, dispatch_uid="update_stock_count")
def update_stock(sender, instance, **kwargs):
 instance.product.stock -= instance.amount

 post_save.disconnect(update_stock, sender=TransactionDetail)
 instance.product.save()
 post_save.connect(update_stock, sender=TransactionDetail)

This is described thoroughly in Disconnect signals for models and reconnect in django, with a more abstract and useful example.

Also see: https://docs.djangoproject.com/en/2.0/topics/signals/#disconnecting-signals in the django docs.

edited Dec 28, 2017 at 12:24

answered Dec 28, 2017 at 12:07

Guglielmo Celata

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2 Comments

Califlower Over a year ago

in order for this to work, you need to have dispatch_uid in the disconnect and connect methods. The return values help you deal with potential errors. ` is_disconnected = post_save.disconnect(update_stock, sender=TransactionDetail, dispatch_uid="update_stock_count") if is_diconnected: instance.product.save() is_reconnected = post_save.connect(update_stock, sender=TransactionDetail, dispatch_uid="update_stock_count") `

Rob Over a year ago

Do not do this. Disconnecting signals could lead to concurrency issues. Especially in a financial application implied by the given code sample! What if two TransactionDetail objects are saved near the same time as each other? If the the first disconnects the signal, as the save() on the second is fired, the signal will not be received as it is disconnected! The correct way to handle this is to use queryset.update() which lets you "save" an object instance without calling save(). See also: stackoverflow.com/a/45895528/4028977

Montaro · Accepted Answer · 2022-06-06 00:04:14Z

6

If you really want to use signals in django please try this:

#import inbuilt user model
from django.contrib.auth.models import User
from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save, sender=User)
def create_profile(sender, **kwargs):
    # write you functionality
    pass

then add default_app_config in the init file

 default_app_config = "give your AppConfig path"

edited Jun 6, 2022 at 0:04

Montaro

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answered Oct 23, 2019 at 5:37

Ankit Patidar

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Comments

Zulu · Accepted Answer · 2023-04-01 01:50:10Z

In fact, the docstring explains the Signals is in django.dispatch.Signal.connect:

def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
    Connect receiver to sender for signal.

    Arguments:

        receiver
            A function or an instance method which is to receive signals.
            Receivers must be hashable objects.

            If weak is True, then receiver must be weak referenceable.

            Receivers must be able to accept keyword arguments.

            If a receiver is connected with a dispatch_uid argument, it
            will not be added if another receiver was already connected
            with that dispatch_uid.

        sender
            The sender to which the receiver should respond. Must either be
            a Python object, or None to receive events from any sender.

        weak
            Whether to use weak references to the receiver. By default, the
            module will attempt to use weak references to the receiver
            objects. If this parameter is false, then strong references will
            be used.

        dispatch_uid
            An identifier used to uniquely identify a particular instance of
            a receiver. This will usually be a string, though it may be
            anything hashable.

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