Cannot assign "<Invoive: Project>": "Invoive.user" must be a "InvoiveCompany" instance

Question

New in Python code stuffs need a help,

Invoices = Parent,
Items = Child,

I need to add This child items in parent, If i directly save this with the code below it creates item for all, Not the specific parent ID. How can i just save this form to a specific ID.

Tried

ItemsForm(request.POST, instance=invoices)

but that didnt work.

def add(request, pk):
datas = Items.objects.all()
invoices = Invoive.objects.get(id=pk)
items = invoices.Items.all()
if request.method == 'POST':
  itemform = ItemsForm(
        request.POST)
if itemform.is_valid():
        itemform.save()
        return redirect(request.META.get('HTTP_REFERER'))