python
36 lines · 1 tab
Priya Sharma
Jan 2026
1 tab
from django.db import transaction
from django.shortcuts import get_object_or_404
from django.http import JsonResponse
from .models import Product
@transaction.atomic
def purchase_product(request, product_id, quantity):
"""Purchase product with stock locking."""
# Lock the product row
product = Product.objects.select_for_update().get(id=product_id)
if product.stock < quantity:
return JsonResponse({'error': 'Insufficient stock'}, status=400)
# Decrement stock
product.stock -= quantity
product.save()
# Create order (simplified)
# Order.objects.create(...)
return JsonResponse({'success': True, 'remaining_stock': product.stock})
def try_reserve_product(product_id):
"""Try to reserve without waiting for lock."""
try:
with transaction.atomic():
product = Product.objects.select_for_update(nowait=True).get(id=product_id)
product.reserved = True
product.save()
return True
except models.DatabaseError:
# Lock not available
return False
1 file · python
Explain with highlit
select_for_update() locks rows until transaction completes, preventing race conditions. I use it for operations requiring read-modify-write atomicity like decrementing stock or updating counters. The lock is released on transaction commit/rollback. For non-blocking behavior, I use nowait=True or skip_locked=True. This is essential for concurrent writes to the same records. I wrap in transaction.atomic() for proper lock management. PostgreSQL and MySQL support this; SQLite has limited support. This prevents lost updates in high-concurrency scenarios.
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