Django post_save() signal implementation

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()

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()