Add unique constraint to baskets

src/purchase/management/commands/generate_dummy_baskets.py

@@ -2,6 +2,7 @@ import random
 from datetime import timedelta
 
 import freezegun
+import numpy as np
 from django.core.management import call_command
 from django.core.management.base import BaseCommand
 from django.utils.timezone import now
@@ -39,10 +40,19 @@ class Command(BaseCommand):
             if random.random() < 0.99:
                 method = random.choices(payment_methods, weights=methods_weights)[0]
             basket = Basket.objects.create(payment_method=method)
+            items_in_basket = int(random.normalvariate(3, 2))
+            if items_in_basket > len(products):
+                items_in_basket = len(products)
+            if items_in_basket < 1:
+                items_in_basket = 1
+            selected_products = np.random.choice(
+                products,
+                size=items_in_basket,
+                replace=False,
+                p=np.asarray(products_weights) / sum(products_weights),
+            )
             items = []
-            item_count = int(random.normalvariate(3, 2))
-            for _ in range(item_count):
-                product: Product = random.choices(products, weights=products_weights)[0]
+            for product in selected_products:
                 items.append(
                     BasketItem(
                         product=product,

src/purchase/migrations/0008_basketitem_unique_product_per_basket.py

@@ -0,0 +1,22 @@
+# Generated by Django 4.0.4 on 2022-05-05 16:46
+
+import django.db.models.expressions
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("purchase", "0007_alter_basketitem_unit_price_cents"),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="basketitem",
+            constraint=models.UniqueConstraint(
+                django.db.models.expressions.F("product"),
+                django.db.models.expressions.F("basket"),
+                name="unique_product_per_basket",
+            ),
+        ),
+    ]

src/purchase/models.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import hashlib
 
 from django.db import models
-from django.db.models import Avg, Count, F, Sum
+from django.db.models import Avg, Count, F, Sum, UniqueConstraint
 from django.db.models.functions import Coalesce
 from django.urls import reverse
 from django.utils.translation import gettext
@@ -217,3 +217,6 @@ class BasketItem(Model):
     class Meta:
         verbose_name = _("basket item")
         verbose_name_plural = _("basket items")
+        constraints = [
+            UniqueConstraint("product", "basket", name="unique_product_per_basket")
+        ]