Fixed serialization issue

src/pretix/base/exporter.py

@@ -68,6 +68,11 @@ class BaseExporter:
 
         :type form_data: dict
         :param form_data: The form data of the export details form
+
+        Note: If you use a ``ModelChoiceField`` (or a ``ModelMultipleChoiceField``), the
+        ``form_data`` will not contain the model instance but only it's primary key (or
+        a list of primary keys) for reasons of internal serialization when using background
+        tasks.
         """
         raise NotImplementedError()  # NOQA
 

src/pretix/control/forms/orders.py

@@ -1,3 +1,6 @@
+from django import forms
+from django.db import models
+
 from pretix.base.forms import I18nModelForm
 from pretix.base.models import Order
 
@@ -6,3 +9,17 @@ class ExtendForm(I18nModelForm):
     class Meta:
         model = Order
         fields = ['expires']
+
+
+class ExporterForm(forms.Form):
+
+    def clean(self):
+        data = super().clean()
+
+        for k, v in data.items():
+            if isinstance(v, models.Model):
+                data[k] = v.pk
+            elif isinstance(v, models.QuerySet):
+                data[k] = [m.pk for m in v]
+
+        return data

src/pretix/control/views/orders.py

@@ -26,7 +26,7 @@ from pretix.base.signals import (
     register_data_exporters, register_payment_providers,
     register_ticket_outputs,
 )
-from pretix.control.forms.orders import ExtendForm
+from pretix.control.forms.orders import ExporterForm, ExtendForm
 from pretix.control.permissions import EventPermissionRequiredMixin
 from pretix.multidomain.urlreverse import build_absolute_uri
 
@@ -391,7 +391,7 @@ class ExportView(EventPermissionRequiredMixin, TemplateView):
         responses = register_data_exporters.send(self.request.event)
         for receiver, response in responses:
             ex = response(self.request.event)
-            ex.form = forms.Form(
+            ex.form = ExporterForm(
                 data=(self.request.POST if self.request.method == 'POST' else None)
             )
             ex.form.fields = ex.export_form_fields

src/pretix/plugins/checkinlists/exporters.py

@@ -6,7 +6,7 @@ from django import forms
 from django.utils.translation import ugettext as _
 
 from pretix.base.exporter import BaseExporter
-from pretix.base.models import Order, OrderPosition
+from pretix.base.models import Order, OrderPosition, Question
 
 
 class BaseCheckinList(BaseExporter):
@@ -65,9 +65,9 @@ class CSVCheckinList(BaseCheckinList):
         output = io.StringIO()
         writer = csv.writer(output, quoting=csv.QUOTE_NONNUMERIC, delimiter=",")
 
-        questions = list(form_data['questions'])
+        questions = list(Question.objects.filter(event=self.event, id__in=form_data['questions']))
         qs = OrderPosition.objects.filter(
-            order__event=self.event, item__in=form_data['items']
+            order__event=self.event, item_id__in=form_data['items']
         ).prefetch_related(
             'answers', 'answers__question'
         ).select_related('order', 'item', 'variation')