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/migrations/0282_taxrule_default.py

@@ -0,0 +1,24 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0281_event_is_remote"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="taxrule",
+            name="default",
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AddConstraint(
+            model_name="taxrule",
+            constraint=models.UniqueConstraint(
+                condition=models.Q(("default", True)),
+                fields=("event",),
+                name="one_default_per_event",
+            ),
+        ),
+    ]

src/pretix/base/migrations/0283_taxrule_default_taxrule_backfill.py

@@ -0,0 +1,60 @@
+# Generated by Django 4.2.17 on 2025-03-28 09:19
+from django.core.cache import cache
+from django.db import migrations, models
+from django.db.models import Count, Exists, OuterRef
+
+
+def set_default_tax_rate(app, schema_editor):
+    Event = app.get_model('pretixbase', 'Event')
+    Event_SettingsStore = app.get_model('pretixbase', 'Event_SettingsStore')
+    TaxRule = app.get_model('pretixbase', 'TaxRule')
+
+    # Handling of events with tax_rate_default set
+    for s in Event_SettingsStore.objects.filter(key="tax_rate_default"):
+        updated = TaxRule.objects.filter(pk=s.value, event_id=s.object_id).update(default=True)
+        if updated:
+            # Delete deprecated settings key
+            s.delete()
+
+            # The default for new events is tax_rule_cancellation=none, but since we do not change behaviour
+            # for existing events without warning, we create a settings entry that matches the old behaviour.
+            Event_SettingsStore.objects.get_or_create(
+                object_id=s.object_id,
+                key="tax_rule_cancellation",
+                defaults={"value": "default"},
+            )
+
+            # We do not need to set tax_rule_payment here since "default" is the default
+
+            cache.delete('hierarkey_{}_{}'.format('event', s.object_id))
+
+    # Handling of events with tax_rate_default not set
+    for e in Event.objects.only("pk").exclude(Exists(TaxRule.objects.filter(default=True, event_id=OuterRef("pk")))):
+        fav_tax_rules = e.tax_rules.annotate(c=Count("item")).order_by("-c", "pk")[:1]
+        if fav_tax_rules:
+            fav_tax_rules[0].default = True
+            fav_tax_rules[0].save()
+
+        # Previously, no tax rule was set for payments, so keep it this way
+        Event_SettingsStore.objects.get_or_create(
+            object=e,
+            key="tax_rule_payment",
+            defaults={"value": "none"},
+        )
+        cache.delete('hierarkey_{}_{}'.format('event', e.pk))
+
+        # We do not need to set tax_rule_cancellation, as "none" is the new system default
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0282_taxrule_default"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            set_default_tax_rate,
+            migrations.RunPython.noop,
+        ),
+    ]