New data model for default tax rule and new options for cancellation fees (#4962)

* New data model for default tax rule

* Remove misleading empty label when field is not optional

* Allow to split cancellation fee

* Fix API and tests

* Update migration

* Update src/tests/api/test_taxrules.py

Co-authored-by: luelista <weller@rami.io>

* Update src/tests/api/test_taxrules.py

Co-authored-by: luelista <weller@rami.io>

* Review note

* Update src/pretix/base/models/tax.py

Co-authored-by: luelista <weller@rami.io>

* Flip API behaviour for default

* Fix failing tests

* Fix failing test

* Split migration

---------

Co-authored-by: luelista <weller@rami.io>

src/pretix/base/models/event.py

@@ -1113,13 +1113,6 @@ class Event(EventMixin, LoggedModel):
                 newname = default_storage.save(fname, fi)
                 s.value = 'file://' + newname
                 settings_to_save.append(s)
-            elif s.key == 'tax_rate_default':
-                try:
-                    if int(s.value) in tax_map:
-                        s.value = tax_map.get(int(s.value)).pk
-                        settings_to_save.append(s)
-                except ValueError:
-                    pass
             elif s.key.startswith('payment_') and s.key.endswith('__restrict_to_sales_channels'):
                 data = other.settings._unserialize(s.value, as_type=list)
                 data = [ident for ident in data if ident in valid_sales_channel_identifers]
@@ -1198,6 +1191,10 @@ class Event(EventMixin, LoggedModel):
                 renderers[pp.identifier] = pp
         return renderers
 
+    @cached_property
+    def cached_default_tax_rule(self):
+        return self.tax_rules.filter(default=True).first()
+
     @cached_property
     def ticket_secret_generators(self) -> dict:
         """

src/pretix/base/models/orders.py

@@ -2373,17 +2373,17 @@ class OrderFee(models.Model):
             self.fee_type, self.value
         )
 
-    def _calculate_tax(self, tax_rule=None):
+    def _calculate_tax(self, tax_rule=None, invoice_address=None):
         if tax_rule:
             self.tax_rule = tax_rule
 
         try:
-            ia = self.order.invoice_address
+            ia = invoice_address or self.order.invoice_address
         except InvoiceAddress.DoesNotExist:
             ia = None
 
-        if not self.tax_rule and self.fee_type == "payment" and self.order.event.settings.tax_rate_default:
-            self.tax_rule = self.order.event.settings.tax_rate_default
+        if not self.tax_rule and self.fee_type == "payment" and self.order.event.settings.tax_rule_payment == "default":
+            self.tax_rule = self.order.event.cached_default_tax_rule
 
         if self.tax_rule:
             tax = self.tax_rule.tax(self.value, base_price_is='gross', invoice_address=ia, force_fixed_gross_price=True)

src/pretix/base/models/tax.py

@@ -377,9 +377,20 @@ class TaxRule(LoggedModel):
                     'if configured above.'),
     )
     custom_rules = models.TextField(blank=True, null=True)
+    default = models.BooleanField(
+        verbose_name=_('Default'),
+        default=False,
+    )
 
     class Meta:
         ordering = ('event', 'rate', 'id')
+        constraints = [
+            models.UniqueConstraint(
+                fields=["event"],
+                condition=models.Q(default=True),
+                name="one_default_per_event",
+            ),
+        ]
 
     class SaleNotAllowed(Exception):
         pass
@@ -394,7 +405,7 @@ class TaxRule(LoggedModel):
             and not OrderFee.objects.filter(tax_rule=self, order__event=self.event).exists()
             and not OrderPosition.all.filter(tax_rule=self, order__event=self.event).exists()
             and not self.event.items.filter(tax_rule=self).exists()
-            and self.event.settings.tax_rate_default != self
+            and not (self.default and self.event.tax_rules.filter(~models.Q(pk=self.pk)).exists())
         )
 
     @classmethod