OrderChangeManager: Fix for change_tax_rule and recalculate_taxes in same transaction (Z#23187299) (#4964)

* Add test case

* Do not overwrite tax_rule in PriceOperation

src/pretix/base/services/orders.py

@@ -2309,7 +2309,7 @@ class OrderChangeManager:
                 op.position.tax_rate = op.price.rate
                 op.position.tax_value = op.price.tax
                 op.position.tax_code = op.price.code
-                op.position.save()
+                op.position.save(update_fields=['price', 'tax_rate', 'tax_value', 'tax_code'])
             elif isinstance(op, self.TaxRuleOperation):
                 if isinstance(op.position, OrderPosition):
                     self.order.log_action('pretix.event.order.changed.tax_rule', user=self.user, auth=self.auth, data={

src/tests/base/test_orders.py

@@ -3243,6 +3243,18 @@ class OrderChangeManagerTests(TestCase):
         assert nop.tax_rate == Decimal('0.00')
         assert nop.tax_value == Decimal('0.00')
 
+    @classscope(attr='o')
+    def test_change_taxrate_and_keep_net(self):
+        self.ocm.change_tax_rule(self.op1, self.tr19)
+        self.ocm.recalculate_taxes(keep='net')
+        self.ocm.commit()
+        self.order.refresh_from_db()
+        nop = self.order.positions.first()
+        assert nop.price == Decimal('25.59')
+        assert nop.tax_rule == self.tr19
+        assert nop.tax_rate == Decimal('19.00')
+        assert nop.tax_value == Decimal('4.09')
+
     @classscope(attr='o')
     def test_change_taxrate_to_country_specific(self):
         self.tr19.eu_reverse_charge = True