Update data instead of creating a new row entry in django model

[Joel Mathew]

I have a page for editing existing data from a model.

The model:

class billitem(models.Model):
code = models.AutoField(primary_key=True, unique=True)
name = models.CharField(max_length=35)
description = models.CharField(max_length=60, null=True)
price = models.DecimalField(decimal_places=2, max_digits=8)

The form:

class BillItem(forms.Form):
code = forms.IntegerField(max_value=100000, disabled=True)
name = forms.CharField(label='Item Name', max_length=32)
description = forms.CharField(
label='Description', max_length=57, required=False)
price = forms.DecimalField(decimal_places=2, max_digits=8)

The view is:

def edit_bill_item(request, itemcode):
if request.method == 'POST':
form = BillItem(request.POST)
code = request.POST.get('code')
name = request.POST.get('name').title()
description = request.POST.get('description')
price = request.POST.get('price')
billitem.objects.filter(code=code).update(
name=name, description=description, price=price)
msg = 'Successfully saved Billing item.'
# form = BillItem(request.POST)
return render(request, 'billing/edititem.html', {'form':
form, 'msg': msg})
else:
itemcode = int(itemcode)
item = get_object_or_404(billitem, code=itemcode)
form = BillItem(initial={
'code': item.code,
'name': item.name,
'description': item.description,
'price': item.price
})
return render(request, 'billing/edititem.html', {'form': form})

The problem is that every time POST is submitted, another entry gets
added in the table, with a new code, instead of updating existing row.

I also tried:

item = billitem(code=code, name=name, description=description, price=price)
item.save()

Which also had the same effect.
How can I handle updation of existing data only within my model and form.

[Jason]

what you should do in your post check is 

           bill_item = BillItem.objects.get(code = itemcode)

form = BillItem(request.POST, instance = bill_item)

This populates the form with the existing model instance and merges with the post data.  you then can run form.save() to persist the update to the db.