Django Signal Handler with Transaction Atomic

Snippet

Django Signal Handler with Transaction Atomic

Django signals enable decoupled communication between components. This expert pattern uses post_save to trigger actions after an order is created. The transaction.atomic context manager ensures that all database operations succeed or fail together, preventing partial updates like confirming an order without reserving stock. The check for product stock prevents overselling in a concurrent environment.

snippet.py
python
from django.db import models, transaction
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.core.mail import send_mail
from django.conf import settings
 
@receiver(post_save, sender='orders.Order')
def order_created_handler(sender, instance, created, **kwargs):
    if not created:
        return
 
    with transaction.atomic(savepoint=True):
        order = instance
        order.status = 'confirmed'
        order.save(update_fields=['status', 'updated_at'])
 
        for item in order.items.select_related('product'):
            product = item.product
            product.stock -= item.quantity
            if product.stock < 0:
                raise ValueError(f'Insufficient stock for {product.name}')
            product.save(update_fields=['stock'])
 
        send_mail(
            subject=f'Order #{order.id} Confirmed',
            message=f'Your order has been confirmed and is being processed.',
            from_email=settings.DEFAULT_FROM_EMAIL,
            recipient_list=[order.customer.email],
            fail_silently=False,
        )

django

Breakdown

@receiver(post_save, sender='orders.Order')

Registers the signal receiver for the Order model's post_save event

if not created: return

Skips processing for updates, only handles new order creation

with transaction.atomic(savepoint=True):

Wraps all database changes in a transaction with savepoint support

update_fields=['status', 'updated_at']

Updates only specific columns to avoid full row locks and trigger less code

select_related('product')

Prevents N+1 queries when accessing product from each order item

if product.stock < 0: raise ValueError(...)

Business rule enforcement within the atomic transaction

Previous snippet Next snippet

More Expert Python exercises →

From your library

Django Signal Handler with Transaction Atomic

Related