[Django]-Django post_save() signal implementation

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()

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.

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"

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.