chore: extracted product batch creation

shop/utils.py

@@ -1,6 +1,6 @@
 from decimal import Decimal
 
-from shop.models import OrderLine, Product, Order, Customer
+from shop.models import OrderLine, Product, Order, Customer, ProductBatch
 
 
 def create_order_line_for_product(product: Product, quantity: Decimal, order: Order):
@@ -61,3 +61,10 @@ def create_order(
     )
 
     return order
+
+
+def delete_product_batch(batch: ProductBatch):
+    product = batch.product
+    product.stock = max(product.stock - batch.quantity, 0)
+    product.save()
+    batch.delete()