Add last_modified property to orders (#907)

src/pretix/api/serializers/order.py

@@ -1,5 +1,3 @@
-from decimal import Decimal
-
 from rest_framework import serializers
 from rest_framework.reverse import reverse
 
@@ -123,18 +121,6 @@ class OrderFeeSerializer(I18nAwareModelSerializer):
         fields = ('fee_type', 'value', 'description', 'internal_type', 'tax_rate', 'tax_value', 'tax_rule')
 
 
-class PaymentFeeLegacyField(serializers.Field):
-    def __init__(self, *args, **kwargs):
-        self.attr = kwargs.pop('attribute')
-        super().__init__(*args, **kwargs)
-
-    def to_representation(self, instance: Order):
-        return str(
-            sum([getattr(f, self.attr) for f in instance.fees.all() if f.fee_type == OrderFee.FEE_TYPE_PAYMENT],
-                Decimal('0.00'))
-        )
-
-
 class OrderSerializer(I18nAwareModelSerializer):
     invoice_address = InvoiceAddressSerializer()
     positions = OrderPositionSerializer(many=True)
@@ -145,7 +131,7 @@ class OrderSerializer(I18nAwareModelSerializer):
         model = Order
         fields = ('code', 'status', 'secret', 'email', 'locale', 'datetime', 'expires', 'payment_date',
                   'payment_provider', 'fees', 'total', 'comment', 'invoice_address', 'positions', 'downloads',
-                  'checkin_attention')
+                  'checkin_attention', 'last_modified')
 
 
 class InlineInvoiceLineSerializer(I18nAwareModelSerializer):