fix

alkatorapi/views.py

@@ -142,10 +142,10 @@ def register_racer(request):
     invoice = Invoice(
         invoice_id=invoice_id,
         user=user,
-        items=[racer],
         total_price=price,
         address=profile.address,
     )
+    invoice.items.set([racer])
     racer.save()
     invoice.save()