Django: How can I protect against concurrent modification of data base entries

UPDATE ... WHERE version = 'version_from_user';

updated = Entry.objects.filter(Q(id=e.id) && Q(version=e.version))
          .update(updated_field=new_value, version=e.version+1)
if not updated:
    raise ConcurrentModificationException()

def save(self):
    if(self.id):
        foo = Foo.objects.get(pk=self.id)
        if(foo.timestamp > self.timestamp):
            raise Exception, "trying to save outdated Foo"
    super(Foo, self).save()