How do I update an already existing row when using ModelForms?

Question

I have a question on how to update an existing row in my database when one of the fields is my primary key. I am using ModelForm and Django-Piston - my main goal here is to have RESTful Post send to my webservice. I am able to have initial Posts be sent correctly (i.e. that Primary key value doesn't exist yet). The problem is when I want to update a value where the Primary key already exists - when I issue a f.is_valid() it fails because "this UniqueIdentifier already exists". How can I do form validation using ModelForms to update an existing row?

My models.py:

from django.db import models
class DeviceModel(models.Model):
    uniqueIdentifier = models.CharField(primary_key=True, max_length=100)
    deviceToken = models.CharField(max_length=100)

forms.py

from django import forms
from models import DeviceModel
class DeviceModelForm(forms.ModelForm):
    class Meta:
        model = DeviceModel

handlers.py

class DeviceHandler(BaseHandler):
allowed_methods = ('POST', 'GET', 'DELETE',)
def create(self, request):
    f = DeviceModelForm(request.POST)
    if f.is_valid():
        new_object = f.save()
        return new_object
    return rc.BAD_REQUEST

urls.py

from django.conf.urls.defaults import *
from piston.resource import Resource
from api.handlers import DeviceHandler

device_handler = Resource(DeviceHandler)

urlpatterns = patterns('',
    (r'^api/$', device_handler, {'emitter_format': 'json'}),
)

Answer 1

To update an existing row (or object in ORM-speak), you have to tell the ModelForm what instance to use when instantiating it:

f = DeviceModelForm(request.POST, instance=myobject)

I'm not sure where you get myobject from using piston, though, but your question seems to imply that you solved that particular problem already.