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/tests/control/test_orders.py

@@ -439,8 +439,37 @@ def test_order_cancel_paid_keep_fee(client, env):
         o.payments.create(state=OrderPayment.PAYMENT_STATE_CONFIRMED, amount=o.total)
         o.status = Order.STATUS_PAID
         o.save()
-        tr7 = o.event.tax_rules.create(rate=Decimal('7.00'))
-        o.event.settings.tax_rate_default = tr7
+        o.event.tax_rules.create(rate=Decimal('7.00'), default=True)
+    client.login(email='dummy@dummy.dummy', password='dummy')
+    client.get('/control/event/dummy/dummy/orders/FOO/transition?status=c')
+    client.post('/control/event/dummy/dummy/orders/FOO/transition', {
+        'status': 'c',
+        'cancellation_fee': '6.00'
+    })
+    with scopes_disabled():
+        o = Order.objects.get(id=env[2].id)
+        assert not o.positions.exists()
+        assert o.all_positions.exists()
+        f = o.fees.get()
+    assert f.fee_type == OrderFee.FEE_TYPE_CANCELLATION
+    assert f.value == Decimal('6.00')
+    assert f.tax_value == Decimal('0.00')
+    assert f.tax_rate == Decimal('0.00')
+    assert f.tax_rule is None
+    assert o.status == Order.STATUS_PAID
+    assert o.total == Decimal('6.00')
+    assert o.pending_sum == Decimal('-8.00')
+
+
+@pytest.mark.django_db
+def test_order_cancel_paid_keep_fee_taxed(client, env):
+    env[0].settings.tax_rule_cancellation = "default"
+    with scopes_disabled():
+        o = Order.objects.get(id=env[2].id)
+        o.payments.create(state=OrderPayment.PAYMENT_STATE_CONFIRMED, amount=o.total)
+        o.status = Order.STATUS_PAID
+        o.save()
+        tr7 = o.event.tax_rules.create(rate=Decimal('7.00'), default=True)
     client.login(email='dummy@dummy.dummy', password='dummy')
     client.get('/control/event/dummy/dummy/orders/FOO/transition?status=c')
     client.post('/control/event/dummy/dummy/orders/FOO/transition', {
@@ -462,6 +491,50 @@ def test_order_cancel_paid_keep_fee(client, env):
     assert o.pending_sum == Decimal('-8.00')
 
 
+@pytest.mark.django_db
+def test_order_cancel_paid_keep_fee_tax_split(client, env):
+    env[0].settings.tax_rule_cancellation = "split"
+    with scopes_disabled():
+        o = Order.objects.get(id=env[2].id)
+        o.payments.create(state=OrderPayment.PAYMENT_STATE_CONFIRMED, amount=o.total)
+        o.status = Order.STATUS_PAID
+        o.save()
+        tr7 = o.event.tax_rules.create(rate=Decimal('7.00'), default=False)
+        tr19 = o.event.tax_rules.create(rate=Decimal('19.00'), default=True)
+        op1 = o.positions.first()
+        op1._calculate_tax(tax_rule=tr7)
+        op1.save()
+        op2 = o.all_positions.last()
+        op2.canceled = False
+        op2._calculate_tax(tax_rule=tr19)
+        op2.save()
+    client.login(email='dummy@dummy.dummy', password='dummy')
+    client.get('/control/event/dummy/dummy/orders/FOO/transition?status=c')
+    client.post('/control/event/dummy/dummy/orders/FOO/transition', {
+        'status': 'c',
+        'cancellation_fee': '6.00'
+    })
+    with scopes_disabled():
+        o = Order.objects.get(id=env[2].id)
+        assert not o.positions.exists()
+        assert o.all_positions.exists()
+        f = o.fees.order_by("-tax_rate")
+    assert len(f) == 2
+    assert f[0].fee_type == OrderFee.FEE_TYPE_CANCELLATION
+    assert f[0].value == Decimal('3.00')
+    assert f[0].tax_value == Decimal('0.48')
+    assert f[0].tax_rate == Decimal('19')
+    assert f[0].tax_rule == tr19
+    assert f[1].fee_type == OrderFee.FEE_TYPE_CANCELLATION
+    assert f[1].value == Decimal('3.00')
+    assert f[1].tax_value == Decimal('0.20')
+    assert f[1].tax_rate == Decimal('7')
+    assert f[1].tax_rule == tr7
+    assert o.status == Order.STATUS_PAID
+    assert o.total == Decimal('6.00')
+    assert o.pending_sum == Decimal('-8.00')
+
+
 @pytest.mark.django_db
 def test_order_cancel_pending_keep_fee(client, env):
     with scopes_disabled():