Save product unit price in basket item

src/purchase/forms.py

@@ -51,13 +51,19 @@ class BasketForm(forms.ModelForm):
     def save(self, commit=True):
         instance: Basket = super().save(commit=True)
         name: str
+        products = {product.id: product for product in Product.objects.all()}
         for name, value in self.cleaned_data.items():
             if name.startswith(PREFIX):
                 product_id = int(name.removeprefix(PREFIX))
+                product = products[product_id]
                 if value > 0:
                     instance.items.update_or_create(
-                        product_id=product_id, defaults={"quantity": value}
+                        product=product,
+                        defaults={
+                            "quantity": value,
+                            "unit_price_cents": product.unit_price_cents,
+                        },
                     )
                 if value == 0:
-                    instance.items.filter(product_id=product_id).delete()
+                    instance.items.filter(product=product).delete()
         return instance

src/purchase/management/commands/generate_dummy_baskets.py

@@ -41,7 +41,10 @@ class Command(BaseCommand):
                 product = random.choice(products)
                 items.append(
                     BasketItem(
-                        product=product, basket=basket, quantity=random.randint(1, 3)
+                        product=product,
+                        basket=basket,
+                        quantity=random.randint(1, 3),
+                        unit_price_cents=product.unit_price_cents,
                     )
                 )
             BasketItem.objects.bulk_create(items)

src/purchase/migrations/0006_basketitem_unit_price_cents.py

@@ -0,0 +1,35 @@
+# Generated by Django 4.0.4 on 2022-04-27 18:30
+
+from django.db import migrations, models
+
+
+def forwards(apps, schema_editor):
+    BasketItem = apps.get_model("purchase", "BasketItem")
+    items = (
+        BasketItem.objects.using(schema_editor.connection.alias)
+        .all()
+        .select_related("product")
+    )
+    for item in items:
+        item.unit_price_cents = item.product.unit_price_cents
+    BasketItem.objects.bulk_update(items, ["unit_price_cents"])
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("purchase", "0005_alter_basket_options_alter_basketitem_options_and_more"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="basketitem",
+            name="unit_price_cents",
+            field=models.PositiveIntegerField(
+                help_text="product's unit price in cents at the time of purchase",
+                null=True,
+                verbose_name="unit price (cents)",
+            ),
+        ),
+        migrations.RunPython(forwards, migrations.RunPython.noop),
+    ]

src/purchase/migrations/0007_alter_basketitem_unit_price_cents.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.0.4 on 2022-04-27 18:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("purchase", "0006_basketitem_unit_price_cents"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="basketitem",
+            name="unit_price_cents",
+            field=models.PositiveIntegerField(
+                help_text="product's unit price in cents at the time of purchase",
+                verbose_name="unit price (cents)",
+            ),
+        ),
+    ]

src/purchase/models.py

@@ -23,7 +23,7 @@ class PaymentMethodQuerySet(models.QuerySet):
             turnover=Coalesce(
                 Sum(
                     F("baskets__items__quantity")
-                    * F("baskets__items__product__unit_price_cents")
+                    * F("baskets__items__unit_price_cents")
                 ),
                 0,
             )
@@ -65,7 +65,8 @@ class ProductQuerySet(models.QuerySet):
     def with_turnover(self):
         return self.annotate(
             turnover=Coalesce(
-                Sum(F("basket_items__quantity") * F("unit_price_cents")), 0
+                Sum(F("basket_items__quantity") * F("basket_items__unit_price_cents")),
+                0,
             )
         )
 
@@ -140,9 +141,7 @@ class Product(Model):
 class BasketQuerySet(models.QuerySet):
     def priced(self) -> BasketQuerySet:
         return self.annotate(
-            price=Coalesce(
-                Sum(F("items__quantity") * F("items__product__unit_price_cents")), 0
-            )
+            price=Coalesce(Sum(F("items__quantity") * F("items__unit_price_cents")), 0)
         )
 
     def average_basket(self) -> float:
@@ -183,9 +182,7 @@ class Basket(Model):
 
 class BasketItemQuerySet(models.QuerySet):
     def priced(self):
-        return self.annotate(
-            price=Coalesce(F("quantity") * F("product__unit_price_cents"), 0)
-        )
+        return self.annotate(price=Coalesce(F("quantity") * F("unit_price_cents"), 0))
 
 
 class BasketItem(Model):
@@ -202,6 +199,10 @@ class BasketItem(Model):
         verbose_name=_("basket"),
     )
     quantity = models.PositiveIntegerField(verbose_name=_("quantity"))
+    unit_price_cents = models.PositiveIntegerField(
+        verbose_name=_("unit price (cents)"),
+        help_text=_("product's unit price in cents at the time of purchase"),
+    )
 
     objects = BasketItemQuerySet.as_manager()