def obj_update(self, bundle, request=None, **kwargs):
    super(MyResource, self).obj_update(bundle, **kwargs)
field_to_update = []
for field_name in self.fields:
    field = self.fields[field_name]
if field.null and field_name in request.POST:
    if request.POST[field_name] is '':
        setattr(bundle.obj, field_name, None)
field_to_update.append(field_name)
bundle.obj.save(update_fields=field_to_update)
return bundle
Using TastyPie to update a ForeignKey field to null