Fix #1440 -- API confusion about creating free orders

src/pretix/api/serializers/order.py

@@ -858,6 +858,9 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
         order.total = sum([p.price for p in order.positions.all()]) + sum([f.value for f in order.fees.all()])
         order.save(update_fields=['total'])
 
+        if order.total == Decimal('0.00') and validated_data.get('status') == Order.STATUS_PAID and not payment_provider:
+            payment_provider = 'free'
+
         if order.total == Decimal('0.00') and validated_data.get('status') != Order.STATUS_PAID:
             order.status = Order.STATUS_PAID
             order.save()

src/tests/api/test_orders.py

@@ -1656,6 +1656,25 @@ def test_order_email_optional(token_client, organizer, event, item, quota, quest
     assert not o.email
 
 
+@pytest.mark.django_db
+def test_order_create_payment_provider_optional_free(token_client, organizer, event, item, quota, question):
+    res = copy.deepcopy(ORDER_CREATE_PAYLOAD)
+    res['positions'][0]['item'] = item.pk
+    res['positions'][0]['answers'][0]['question'] = question.pk
+    res['positions'][0]['price'] = '0.00'
+    res['positions'][0]['status'] = 'p'
+    del res['payment_provider']
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orders/'.format(
+            organizer.slug, event.slug
+        ), format='json', data=res
+    )
+    assert resp.status_code == 201
+    with scopes_disabled():
+        o = Order.objects.get(code=resp.data['code'])
+        assert not o.payments.exists()
+
+
 @pytest.mark.django_db
 def test_order_create_payment_info_optional(token_client, organizer, event, item, quota, question):
     res = copy.deepcopy(ORDER_CREATE_PAYLOAD)