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)