Restock product inventory when an order is cancelled

checkout() decrements stock_quantity/increments sold_count per order
item, but cancellation never reversed it, so a cancelled order's stock
stayed permanently reduced. Add _restock_order_items(), called from
_transition() on the CANCELLED transition, using F() expressions to
reverse both counters atomically; wrap _transition() itself in
@transaction.atomic so the status change and restock can't partially
apply.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/orders/services.py

@@ -1,11 +1,14 @@
 from itertools import groupby
 
 from django.db import transaction
+from django.db.models import F
 from django.utils import timezone
 from rest_framework.exceptions import ValidationError
 
 from utils.exceptions import Conflict
 
+from apps.catalog.models import Product
+
 from .models import Cart, Notification, Order, OrderGroup, OrderItem, OrderStatusLog
 
 
@@ -136,6 +139,16 @@ def checkout(user, address, delivery_type, scheduled_at, payment_method, notes):
     return order_group
 
 
+def _restock_order_items(order):
+    """Reverses the stock_quantity/sold_count decrement made at checkout (see checkout())."""
+    for item in order.items.select_related('product').filter(product__isnull=False):
+        Product.objects.filter(pk=item.product_id).update(
+            stock_quantity=F('stock_quantity') + item.quantity,
+            sold_count=F('sold_count') - item.quantity,
+        )
+
+
+@transaction.atomic
 def _transition(order, to_status, *, changed_by=None, note='', **extra_fields):
     from_status = order.status
     order.status = to_status
@@ -147,6 +160,7 @@ def _transition(order, to_status, *, changed_by=None, note='', **extra_fields):
     OrderStatusLog.objects.create(order=order, from_status=from_status, to_status=to_status, changed_by=changed_by, note=note)
 
     if to_status == Order.Status.CANCELLED:
+        _restock_order_items(order)
         notify(
             recipient=order.store.owner,
             type=Notification.Type.ORDER_CANCELLED,

apps/orders/tests/test_status_transitions.py

@@ -50,6 +50,30 @@ class StatusTransitionTests(OrdersTestCase):
         self.order.refresh_from_db()
         self.assertEqual(self.order.status, Order.Status.CANCELLED)
 
+    def test_cancelling_an_order_restocks_its_products(self):
+        self.product1.refresh_from_db()
+        stock_after_checkout = self.product1.stock_quantity
+        sold_after_checkout = self.product1.sold_count
+        self.assertEqual(stock_after_checkout, 49)  # base.py seeds 50, checkout bought 1
+        self.assertEqual(sold_after_checkout, 1)
+
+        self.client.force_authenticate(user=self.customer)
+        response = self.client.post(f'/api/v1/orders/{self.order.uuid}/cancel/')
+        self.assertEqual(response.status_code, 200)
+
+        self.product1.refresh_from_db()
+        self.assertEqual(self.product1.stock_quantity, stock_after_checkout + 1)
+        self.assertEqual(self.product1.sold_count, sold_after_checkout - 1)
+
+    def test_cancelling_after_confirm_also_restocks(self):
+        self._as_seller()
+        self.client.post(f'/api/v1/seller/orders/{self.order.uuid}/confirm/')
+        self.client.post(f'/api/v1/seller/orders/{self.order.uuid}/cancel/')
+
+        self.product1.refresh_from_db()
+        self.assertEqual(self.product1.stock_quantity, 50)
+        self.assertEqual(self.product1.sold_count, 0)
+
     def test_cannot_cancel_after_handed_to_courier(self):
         self._as_seller()
         for action in ('confirm', 'mark-ready', 'mark-shipped'):