Fixes various errors in discount calculation, and adds tests for these

registrasion/controllers/cart.py

@@ -183,7 +183,12 @@ class CartController(object):
         # Delete the existing entries.
         rego.DiscountItem.objects.filter(cart=self.cart).delete()
 
-        for item in self.cart.productitem_set.all():
+        # The highest-value discounts will apply to the highest-value
+        # products first.
+        product_items = self.cart.productitem_set.all()
+        product_items = product_items.order_by('product__price')
+        product_items = reversed(product_items)
+        for item in product_items:
             self._add_discount(item.product, item.quantity)
 
     def _add_discount(self, product, quantity):
@@ -202,9 +207,7 @@ class CartController(object):
             # Get the count of past uses of this discount condition
             # as this affects the total amount we're allowed to use now.
             past_uses = rego.DiscountItem.objects.filter(
-                cart__active=False,
                 discount=discount.discount,
-                product=product,
             )
             agg = past_uses.aggregate(Sum("quantity"))
             past_uses = agg["quantity__sum"]

registrasion/tests/test_cart.py

@@ -82,7 +82,18 @@ class RegistrationCartTestCase(SetTimeMixin, TestCase):
             limit_per_user=10,
             order=10,
         )
-        cls.PROD_2.save()
+        cls.PROD_3.save()
+
+        cls.PROD_4 = rego.Product.objects.create(
+            name="Product 4",
+            description="This is a test product. It costs $5. "
+                        "A user may have 10 of them.",
+            category=cls.CAT_2,
+            price=Decimal("5.00"),
+            limit_per_user=10,
+            order=10,
+        )
+        cls.PROD_4.save()
 
     @classmethod
     def make_ceiling(cls, name, limit=None, start_time=None, end_time=None):

registrasion/tests/test_discount.py

@@ -29,7 +29,10 @@ class DiscountTestCase(RegistrationCartTestCase):
         return discount
 
     @classmethod
-    def add_discount_prod_1_includes_cat_2(cls, amount=Decimal(100)):
+    def add_discount_prod_1_includes_cat_2(
+            cls,
+            amount=Decimal(100),
+            quantity=2):
         discount = rego.IncludedProductDiscount.objects.create(
             description="PROD_1 includes CAT_2 " + str(amount) + "%",
         )
@@ -40,7 +43,7 @@ class DiscountTestCase(RegistrationCartTestCase):
             discount=discount,
             category=cls.CAT_2,
             percentage=amount,
-            quantity=2
+            quantity=quantity,
         ).save()
         return discount
 
@@ -169,3 +172,31 @@ class DiscountTestCase(RegistrationCartTestCase):
 
         discount_items = list(cart.cart.discountitem_set.all())
         self.assertEqual(2, discount_items[0].quantity)
+
+    def test_category_discount_applies_once_per_category(self):
+        self.add_discount_prod_1_includes_cat_2(quantity=1)
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_1, 1)
+
+        # Add two items from category 2
+        cart.add_to_cart(self.PROD_3, 1)
+        cart.add_to_cart(self.PROD_4, 1)
+
+        discount_items = list(cart.cart.discountitem_set.all())
+        # There is one discount, and it should apply to one item.
+        self.assertEqual(1, len(discount_items))
+        self.assertEqual(1, discount_items[0].quantity)
+
+    def test_category_discount_applies_to_highest_value(self):
+        self.add_discount_prod_1_includes_cat_2(quantity=1)
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_1, 1)
+
+        # Add two items from category 2, add the less expensive one first
+        cart.add_to_cart(self.PROD_4, 1)
+        cart.add_to_cart(self.PROD_3, 1)
+
+        discount_items = list(cart.cart.discountitem_set.all())
+        # There is one discount, and it should apply to the more expensive.
+        self.assertEqual(1, len(discount_items))
+        self.assertEqual(self.PROD_3, discount_items[0].product)