Fixed serialization issue

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