Improve order change

src/pretix/base/services/orders.py

@@ -1736,7 +1736,7 @@ class OrderChangeManager:
         if position.issued_gift_cards.exists():
             raise OrderError(self.error_messages['gift_card_change'])
 
-        self._totaldiff_guesstimate += price.gross - position.price
+        self._totaldiff_guesstimate += price.gross - position.price + position.price_includes_rounding_correction
 
         if self.order.event.settings.invoice_include_free or price.gross != Decimal('0.00') or position.price != Decimal('0.00'):
             self._invoice_dirty = True
@@ -2362,10 +2362,15 @@ class OrderChangeManager:
                     'new_price': op.price.gross
                 })
                 position.price = op.price.gross
+                position.price_includes_rounding_correction = Decimal("0.00")
                 position.tax_rate = op.price.rate
                 position.tax_value = op.price.tax
+                position.tax_value_includes_rounding_correction = Decimal("0.00")
                 position.tax_code = op.price.code
-                position.save(update_fields=['price', 'tax_rate', 'tax_value', 'tax_code'])
+                position.save(update_fields=[
+                    'price', 'price_includes_rounding_correction', 'tax_rate', 'tax_value',
+                    'tax_value_includes_rounding_correction', 'tax_code'
+                ])
             elif isinstance(op, self.TaxRuleOperation):
                 if isinstance(op.position, OrderPosition):
                     position = position_cache.setdefault(op.position.pk, op.position)

src/tests/base/test_orders.py

@@ -1568,6 +1568,63 @@ class OrderChangeManagerTests(TestCase):
         assert round_decimal(self.op1.price * (1 - 100 / (100 + self.op1.tax_rate))) == self.op1.tax_value
         assert self.order.total == self.op1.price + self.op2.price
 
+    @classscope(attr='o')
+    def test_change_price_with_rounding_change_impossible(self):
+        # Order starts with 2*100€ tickets, but rounding corrects it to 199€. Then, the user tries to force both prices
+        # to 100€. No luck.
+        self.order.status = Order.STATUS_PAID
+        self.order.tax_rounding_mode = "sum_by_net"
+        self.order.save()
+        self.op1.price = Decimal("100.00")
+        self.op1._calculate_tax(tax_rule=self.tr19)
+        self.op1.save()
+        self.op2.price = Decimal("100.00")
+        self.op2._calculate_tax(tax_rule=self.tr19)
+        self.op2.save()
+        self.order.refresh_from_db()
+        self.ocm.regenerate_secret(self.op1)
+        self.ocm.commit()  # Force re-rounding
+        self.order.refresh_from_db()
+        self.ocm = OrderChangeManager(self.order, None)
+        assert self.order.total == Decimal("199.99")
+
+        self.ocm.change_price(self.op1, Decimal('100.00'))
+        self.ocm.change_price(self.op2, Decimal('100.00'))
+        self.ocm.commit()
+        self.op1.refresh_from_db()
+        self.op2.refresh_from_db()
+        self.order.refresh_from_db()
+        assert self.order.total == Decimal("199.99")
+        assert self.op1.price == Decimal('99.99')
+        assert self.op2.price == Decimal('100.00')
+
+    @classscope(attr='o')
+    def test_change_price_with_rounding_change_autocorrected(self):
+        self.order.status = Order.STATUS_PAID
+        self.order.tax_rounding_mode = "sum_by_net"
+        self.order.save()
+        self.op1.price = Decimal("0.00")
+        self.op1._calculate_tax(tax_rule=self.tr19)
+        self.op1.save()
+        self.op2.price = Decimal("100.00")
+        self.op2._calculate_tax(tax_rule=self.tr19)
+        self.op2.save()
+        self.order.refresh_from_db()
+        self.ocm.regenerate_secret(self.op1)
+        self.ocm.commit()  # Force re-rounding
+        self.order.refresh_from_db()
+        self.ocm = OrderChangeManager(self.order, None)
+        assert self.order.total == Decimal("100.00")
+
+        self.ocm.change_price(self.op1, Decimal('100.00'))
+        self.ocm.commit()
+        self.op1.refresh_from_db()
+        self.op2.refresh_from_db()
+        self.order.refresh_from_db()
+        assert self.order.total == Decimal("199.99")
+        assert self.op1.price == Decimal('99.99')
+        assert self.op2.price == Decimal('100.00')
+
     @classscope(attr='o')
     def test_change_price_net_success(self):
         self.tr7.price_includes_tax = False

src/tests/base/test_pricing_rounding.py

@@ -219,3 +219,29 @@ def test_round_currency_without_decimals():
     assert sum(l.price for l in lines) == Decimal("49990.00")
     assert sum(l.tax_value for l in lines) == Decimal("7982.00")
     assert sum(l.price - l.tax_value for l in lines) == Decimal("42008.00")
+
+
+@pytest.mark.django_db
+@pytest.mark.parametrize("rounding_mode", [
+    "sum_by_net",
+    "sum_by_net_keep_gross",
+])
+def test_do_not_touch_free(rounding_mode):
+    l1 = OrderPosition(
+        price=Decimal("0.00"),
+    )
+    l1._calculate_tax(tax_rule=TaxRule(rate=Decimal("7.00")), invoice_address=InvoiceAddress())
+    l2 = OrderPosition(
+        price=Decimal("23.00"),
+    )
+    l2._calculate_tax(tax_rule=TaxRule(rate=Decimal("7.00")), invoice_address=InvoiceAddress())
+    apply_rounding(rounding_mode, "EUR", [l1, l2])
+    assert l2.price == Decimal("23.01")
+    assert l2.price_includes_rounding_correction == Decimal("0.01")
+    assert l2.tax_value == Decimal("1.51")
+    assert l2.tax_value_includes_rounding_correction == Decimal("0.01")
+    assert l2.tax_rate == Decimal("7.00")
+    assert l1.price == Decimal("0.00")
+    assert l1.price_includes_rounding_correction == Decimal("0.00")
+    assert l1.tax_value == Decimal("0.00")
+    assert l1.tax_value_includes_rounding_correction == Decimal("0.00")