Serializer not writing individual object from queryset

Question

I am using a ModelForm to capture some data for a model and, while I want this data saved to a db, I also want to export it to an XML file for transformation/use in an external system.

Please see below for an example:

def warranty(request):
    WarrantyFormSet = modelformset_factory(Warranty, form=WarrantyForm)
    if request.method == 'POST':
        formset = WarrantyFormSet(request.POST, request.FILES)
        if formset.is_valid():
            new = formset.save(commit=False)
            out = open("file.xml", "w")
            XMLSerializer = serializers.get_serializer("xml")
            xml_serializer = XMLSerializer()

            for n in new:
                xml_serializer.serialize(Warranty.objects.all(), stream=out)
                #.filter(id = n.id)
                n.save()

            return HttpResponse(xml_serializer.serialize(Warranty.objects.filter(id = n.id)))
    else:
        formset = WarrantyFormSet(queryset = Warranty.objects.none())
    return render(request,'warranty.html', {'formset': formset})

The object is serializing appropriately in the HttpResponse (ie I can see an acceptable XML output), but there is no output in the XML file itself. If I remove the QuerySet filter (i.e. call .all()), then the XML file will correctly have all objects related to the Warranty model. It seems strange that the behaviour is fine in one circumstance in not theother, so I can't trouble shoot any further.

Answer 1

The issue is that you're trying to serialize the objects before they are actually saved to the database - so the queryset passed to the serializer will not contain this new item. Change the order around:

for n in new:
    n.save()
    xml_serializer.serialize(Warranty.objects.filter(id=n.id), stream=out)

Also note that your current logic write a new serialization to the file inside your for loop - which I am not sure is what you want. You could instead serialize everything at once like so:

if request.method == 'POST':
    formset = WarrantyFormSet(request.POST, request.FILES)
    if formset.is_valid():
        new_items = formset.save()  # commit = True since you don't need to change anything before saving
        new_item_ids = [n.id for n in new_items]
        with open("file.xml", "w") as out:
            XMLSerializer = serializers.get_serializer("xml")
            xml_serializer = XMLSerializer()
            xml_serializer.serialize(
                Warranty.objects.filter(id__in=new_item_ids), stream=out)