feat: update product stock when a product batch is created

shop/api/v1/viewsets.py

@@ -100,6 +100,18 @@ class ProductBatchViewSet(ModelViewSet):
     queryset = ProductBatch.objects.all()
     permission_classes = (IsAdminUser,)
 
+    def perform_create(self, serializer):
+        instance = serializer.save()
+        product = instance.product
+        product.stock += instance.quantity
+        product.save()
+
+    def perform_destroy(self, instance):
+        product = instance.product
+        product.stock = max(product.stock - instance.quantity, 0)
+        product.save()
+        instance.delete()
+
 
 class CustomerViewSet(ModelViewSet):
     serializer_class = CustomerSerializer