Allow more decimal places for tax rates (#6092)

* Allow more decimal places for tax rates

* Fix normalization

* More tax rate handling

* Add uncommitted tests

* Rebase migration

* Update src/pretix/base/migrations/0304_tax_rate_decimals.py

Co-authored-by: robbi5 <richt@rami.io>

---------

Co-authored-by: robbi5 <richt@rami.io>

src/pretix/api/serializers/__init__.py

@@ -20,6 +20,7 @@
 # <https://www.gnu.org/licenses/>.
 #
 import json
+import re
 
 from django.db.models import prefetch_related_objects
 from rest_framework import serializers
@@ -135,3 +136,30 @@ class SalesChannelMigrationMixin:
         else:
             value["sales_channels"] = value["limit_sales_channels"]
         return value
+
+
+class CompatDecimalField(serializers.DecimalField):
+    """
+    Historically, pretix recorded tax rates as decimals with two places. Today, pretix supports tax rates with up to
+    four places. Since our API outputs decimals with the stored precision, this would have changed the API output from
+    "19.00" to "19.0000" without warning. While this is semantically the same thing, we need to assume some pretix API
+    users might run into trouble, either because they treat the value as a string and then map something
+    (e.g. ``if tax_rate == "19.00"``) or process it with a language where this is a significant difference. For example,
+    while in Python ``Decimal("19.00") == Decimal("19.0000")`` is true, in Java
+    ``(new BigDecimal("19.00")).equals(new BigDecimal("19.0000"))`` is false and only
+    ``(new BigDecimal("19.00")).compareTo(new BigDecimal("19.0000")) == 0`` is true.
+
+    Therefore, we stay backwards compatible by outputting two decimal places *as long as the trailing digits are zero-valued.
+    """
+
+    regex = re.compile(r"^([0-9]+\.[0-9]{2})0+$")
+
+    def to_representation(self, value):
+        if self.localize:
+            raise ValueError("localization not supported")
+        value = super().to_representation(value)
+        if value and "." not in value:
+            return f"{value}.00"
+        if m := self.regex.match(value):
+            return m.group(1)
+        return value

src/pretix/api/serializers/event.py

@@ -48,7 +48,7 @@ from rest_framework.fields import ChoiceField, Field
 from rest_framework.relations import SlugRelatedField
 
 from pretix.api.serializers import (
-    CompatibleJSONField, SalesChannelMigrationMixin,
+    CompatDecimalField, CompatibleJSONField, SalesChannelMigrationMixin,
 )
 from pretix.api.serializers.fields import PluginsField
 from pretix.api.serializers.i18n import I18nAwareModelSerializer
@@ -681,6 +681,7 @@ class TaxRuleSerializer(CountryFieldMixin, I18nAwareModelSerializer):
         required=False,
         allow_null=True,
     )
+    rate = CompatDecimalField(max_digits=7, decimal_places=4)
 
     class Meta:
         model = TaxRule

src/pretix/api/serializers/item.py

@@ -42,7 +42,9 @@ from django.utils.functional import cached_property, lazy
 from django.utils.translation import gettext_lazy as _
 from rest_framework import serializers
 
-from pretix.api.serializers import SalesChannelMigrationMixin
+from pretix.api.serializers import (
+    CompatDecimalField, SalesChannelMigrationMixin,
+)
 from pretix.api.serializers.event import MetaDataField
 from pretix.api.serializers.fields import UploadedFileField
 from pretix.api.serializers.i18n import I18nAwareModelSerializer
@@ -276,10 +278,10 @@ class ItemAddOnSerializer(serializers.ModelSerializer):
         return value
 
 
-class ItemTaxRateField(serializers.Field):
+class ItemTaxRateField(CompatDecimalField):
     def to_representation(self, i):
         if i.tax_rule:
-            return str(Decimal(i.tax_rule.rate))
+            return super().to_representation(Decimal(i.tax_rule.rate))
         else:
             return str(Decimal('0.00'))
 
@@ -289,7 +291,7 @@ class ItemSerializer(SalesChannelMigrationMixin, I18nAwareModelSerializer):
     bundles = InlineItemBundleSerializer(many=True, required=False)
     variations = InlineItemVariationSerializer(many=True, required=False)
     program_times = InlineItemProgramTimeSerializer(many=True, required=False)
-    tax_rate = ItemTaxRateField(source='*', read_only=True)
+    tax_rate = ItemTaxRateField(source='*', read_only=True, max_digits=7, decimal_places=4)
     meta_data = MetaDataField(required=False, source='*')
     picture = UploadedFileField(required=False, allow_null=True, allowed_types=(
         'image/png', 'image/jpeg', 'image/gif'

src/pretix/api/serializers/order.py

@@ -41,7 +41,7 @@ from rest_framework.exceptions import ValidationError
 from rest_framework.relations import SlugRelatedField
 from rest_framework.reverse import reverse
 
-from pretix.api.serializers import CompatibleJSONField
+from pretix.api.serializers import CompatDecimalField, CompatibleJSONField
 from pretix.api.serializers.event import SubEventSerializer
 from pretix.api.serializers.forms import form_field_to_serializer_field
 from pretix.api.serializers.i18n import I18nAwareModelSerializer
@@ -591,6 +591,7 @@ class OrderPositionSerializer(I18nAwareModelSerializer):
     country = CompatibleCountryField(source='*')
     attendee_name = serializers.CharField(required=False)
     plugin_data = OrderPositionPluginDataField(source='*', allow_null=True, read_only=True)
+    tax_rate = CompatDecimalField(max_digits=7, decimal_places=4)
 
     class Meta:
         list_serializer_class = OrderPositionListSerializer
@@ -747,6 +748,8 @@ class OrderPaymentDateField(serializers.DateField):
 
 
 class OrderFeeSerializer(I18nAwareModelSerializer):
+    tax_rate = CompatDecimalField(max_digits=7, decimal_places=4)
+
     class Meta:
         model = OrderFee
         fields = ('id', 'fee_type', 'value', 'description', 'internal_type', 'tax_rate', 'tax_value', 'tax_rule',
@@ -1911,6 +1914,7 @@ class InlineInvoiceLineSerializer(I18nAwareModelSerializer):
     position = LinePositionField(read_only=True)
     event_date_from = serializers.DateTimeField(read_only=True, source="period_start")
     event_date_to = serializers.DateTimeField(read_only=True, source="period_end")
+    tax_rate = CompatDecimalField(max_digits=7, decimal_places=4)
 
     class Meta:
         model = InvoiceLine
@@ -1994,6 +1998,7 @@ class BlockedTicketSecretSerializer(I18nAwareModelSerializer):
 
 class TransactionSerializer(I18nAwareModelSerializer):
     order = serializers.SlugRelatedField(slug_field="code", read_only=True)
+    tax_rate = CompatDecimalField(max_digits=7, decimal_places=4)
 
     class Meta:
         model = Transaction