Subclass MultipleChoiceField to serialize to list (#1605)

* Subclass MultipleChoiceField to serialize to list

* Rename pretix.api.serializers.MultipleChoiceField to ListMultipleChoiceField

* Keep order in ListMultipleChoiceField

src/pretix/api/serializers/fields.py

@@ -0,0 +1,29 @@
+from collections import OrderedDict
+
+from rest_framework import serializers
+
+
+def remove_duplicates_from_list(data):
+    return list(OrderedDict.fromkeys(data))
+
+
+class ListMultipleChoiceField(serializers.MultipleChoiceField):
+    def to_internal_value(self, data):
+        if isinstance(data, str) or not hasattr(data, '__iter__'):
+            self.fail('not_a_list', input_type=type(data).__name__)
+        if not self.allow_empty and len(data) == 0:
+            self.fail('empty')
+
+        internal_value_data = [
+            super(serializers.MultipleChoiceField, self).to_internal_value(item)
+            for item in data
+        ]
+
+        return remove_duplicates_from_list(internal_value_data)
+
+    def to_representation(self, value):
+        representation_data = [
+            self.choice_strings_to_values.get(str(item), item) for item in value
+        ]
+
+        return remove_duplicates_from_list(representation_data)

src/pretix/base/settings.py

@@ -19,6 +19,7 @@ from i18nfield.forms import I18nFormField, I18nTextarea, I18nTextInput
 from i18nfield.strings import LazyI18nString
 from rest_framework import serializers
 
+from pretix.api.serializers.fields import ListMultipleChoiceField
 from pretix.api.serializers.i18n import I18nField
 from pretix.base.models.tax import TaxRule
 from pretix.base.reldate import (
@@ -639,7 +640,7 @@ DEFAULTS = {
     'locales': {
         'default': json.dumps([settings.LANGUAGE_CODE]),
         'type': list,
-        'serializer_class': serializers.MultipleChoiceField,
+        'serializer_class': ListMultipleChoiceField,
         'serializer_kwargs': dict(
             choices=settings.LANGUAGES,
             required=True,

src/tests/api/test_events.py

@@ -1032,6 +1032,32 @@ def test_patch_event_settings(token_client, organizer, event):
     )
     assert resp.status_code == 405
 
+    locales = event.settings.locales
+
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/settings/'.format(organizer.slug, event.slug),
+        {
+            'locales': event.settings.locales + ['de', 'de-informal'],
+        },
+        format='json'
+    )
+    assert resp.status_code == 200
+    assert set(resp.data['locales']) == set(locales + ['de', 'de-informal'])
+    event.settings.flush()
+    assert set(event.settings.locales) == set(locales + ['de', 'de-informal'])
+
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/settings/'.format(organizer.slug, event.slug),
+        {
+            'locales': locales,
+        },
+        format='json'
+    )
+    assert resp.status_code == 200
+    assert set(resp.data['locales']) == set(locales)
+    event.settings.flush()
+    assert set(event.settings.locales) == set(locales)
+
 
 @pytest.mark.django_db
 def test_patch_event_settings_validation(token_client, organizer, event):