API: Add discount to order position serializer

doc/api/resources/orders.rst

@@ -178,6 +178,7 @@ tax_rule                              integer                    The ID of the u
 secret                                string                     Secret code printed on the tickets for validation
 addon_to                              integer                    Internal ID of the position this position is an add-on for (or ``null``)
 subevent                              integer                    ID of the date inside an event series this position belongs to (or ``null``).
+discount                              integer                    ID of a discount that has been used during the creation of this position in some way (or ``null``).
 pseudonymization_id                   string                     A random ID, e.g. for use in lead scanning apps
 checkins                              list of objects            List of **successful** check-ins with this ticket
 ├ id                                  integer                    Internal ID of the check-in event
@@ -371,6 +372,7 @@ List of all orders
                 "secret": "z3fsn8jyufm5kpk768q69gkbyr5f4h6w",
                 "addon_to": null,
                 "subevent": null,
+                "discount": null,
                 "pseudonymization_id": "MQLJvANO3B",
                 "seat": null,
                 "checkins": [
@@ -546,6 +548,7 @@ Fetching individual orders
             "secret": "z3fsn8jyufm5kpk768q69gkbyr5f4h6w",
             "addon_to": null,
             "subevent": null,
+            "discount": null,
             "pseudonymization_id": "MQLJvANO3B",
             "seat": null,
             "checkins": [
@@ -1487,6 +1490,7 @@ List of all order positions
             "tax_rule": null,
             "tax_value": "0.00",
             "secret": "z3fsn8jyufm5kpk768q69gkbyr5f4h6w",
+            "discount": null,
             "pseudonymization_id": "MQLJvANO3B",
             "seat": null,
             "addon_to": null,
@@ -1597,6 +1601,7 @@ Fetching individual positions
         "secret": "z3fsn8jyufm5kpk768q69gkbyr5f4h6w",
         "addon_to": null,
         "subevent": null,
+        "discount": null,
         "pseudonymization_id": "MQLJvANO3B",
         "seat": null,
         "checkins": [

src/pretix/api/serializers/order.py

@@ -410,13 +410,13 @@ class OrderPositionSerializer(I18nAwareModelSerializer):
     class Meta:
         model = OrderPosition
         fields = ('id', 'order', 'positionid', 'item', 'variation', 'price', 'attendee_name', 'attendee_name_parts',
-                  'company', 'street', 'zipcode', 'city', 'country', 'state',
+                  'company', 'street', 'zipcode', 'city', 'country', 'state', 'discount',
                   'attendee_email', 'voucher', 'tax_rate', 'tax_value', 'secret', 'addon_to', 'subevent', 'checkins',
                   'downloads', 'answers', 'tax_rule', 'pseudonymization_id', 'pdf_data', 'seat', 'canceled')
         read_only_fields = (
             'id', 'order', 'positionid', 'item', 'variation', 'price', 'voucher', 'tax_rate', 'tax_value', 'secret',
             'addon_to', 'subevent', 'checkins', 'downloads', 'answers', 'tax_rule', 'pseudonymization_id', 'pdf_data',
-            'seat', 'canceled'
+            'seat', 'canceled', 'discount',
         )
 
     def __init__(self, *args, **kwargs):

src/tests/api/test_order_create.py

@@ -341,6 +341,7 @@ def test_order_create_simulate(token_client, organizer, event, item, quota, ques
                 'tax_value': '0.00',
                 'addon_to': None,
                 'subevent': None,
+                'discount': None,
                 'checkins': [],
                 'downloads': [],
                 'answers': [
@@ -409,13 +410,13 @@ def test_order_create_positionids_addons_simulated(token_client, organizer, even
         {'id': 0, 'order': '', 'positionid': 1, 'item': item.pk, 'variation': None, 'price': '23.00',
          'attendee_name': 'Peter', 'attendee_name_parts': {'full_name': 'Peter', '_scheme': 'full'}, 'company': None,
          'street': None, 'zipcode': None, 'city': None, 'country': None, 'state': None, 'attendee_email': None,
-         'voucher': None, 'tax_rate': '0.00', 'tax_value': '0.00',
+         'voucher': None, 'tax_rate': '0.00', 'tax_value': '0.00', 'discount': None,
          'addon_to': None, 'subevent': None, 'checkins': [], 'downloads': [], 'answers': [], 'tax_rule': None,
          'pseudonymization_id': 'PREVIEW', 'seat': None, 'canceled': False},
         {'id': 0, 'order': '', 'positionid': 2, 'item': item.pk, 'variation': None, 'price': '23.00',
          'attendee_name': 'Peter', 'attendee_name_parts': {'full_name': 'Peter', '_scheme': 'full'}, 'company': None,
          'street': None, 'zipcode': None, 'city': None, 'country': None, 'state': None, 'attendee_email': None,
-         'voucher': None, 'tax_rate': '0.00', 'tax_value': '0.00',
+         'voucher': None, 'tax_rate': '0.00', 'tax_value': '0.00', 'discount': None,
          'addon_to': 1, 'subevent': None, 'checkins': [], 'downloads': [], 'answers': [], 'tax_rule': None,
          'pseudonymization_id': 'PREVIEW', 'seat': None, 'canceled': False}
     ]

src/tests/api/test_orders.py

@@ -157,6 +157,7 @@ TEST_ORDERPOSITION_RES = {
     "attendee_name": "Peter",
     "attendee_email": None,
     "voucher": None,
+    "discount": None,
     "tax_rate": "0.00",
     "tax_value": "0.00",
     "tax_rule": None,