Django snippets

#!/usr/bin/env python
# -*- coding: utf-8 -
#
#
# Look here: https://bitbucket.org/depaolim/optlock
# The snippet of Marco De Paoli is much better than this one! :-)
#
#
#
# @contact: marcoberi@gmail.com
# @version: 1.0
# @license: MIT http://www.opensource.org/licenses/mit-license.php
#

from django.db import models, connection
from django.utils.translation import ugettext as _
from django.contrib import admin

class ExceptionNewVersionInDb(Exception):
    def __init__(self, obj, version, code=None, params=None):
        from django.utils.encoding import force_unicode
        self.message = force_unicode("{0} ver {1}: {2}".format(unicode(obj),
            version, _("saved by another user")))

    def __str__(self):
        return repr(self.message)

    def __repr__(self):
        return 'ExceptionNewVersionInDb(%s)' % repr(self.message)

class Model(models.Model):
    _version_opt_lock = models.IntegerField(
            blank = True, null = True,
            default = 0,
            verbose_name = _("Version"))

    def get_version_in_db(self):
        if self.pk:
            cursor = connection.cursor()
            raw_query = 'select _version_opt_lock from "{0}" where "{1}" = '.format(
                self._meta.db_table, self._meta.pk.attname)
            cursor.execute(raw_query + "%s", (self.pk,))
            retval = cursor.fetchone()
            return retval[0]
        return 0

    def save(self, *args, **kw_args):
        if self.pk:
            version_in_db = self.get_version_in_db()
            if self._version_opt_lock != version_in_db:
                raise ExceptionNewVersionInDb(self, version_in_db)
            self._version_opt_lock = version_in_db + 1
        else:
            self._version_opt_lock = 1
        super(Model, self).save(*args, **kw_args)

    class Meta:
        abstract = True

class ModelAdmin(admin.ModelAdmin):
    def formfield_for_dbfield(self, db_field, *args, **kwargs):
        formfield = super(ModelAdmin, self).formfield_for_dbfield(db_field, *args, **kwargs)
        if (db_field.name == "_version_opt_lock"):
            formfield.widget.attrs["readonly"] = "readonly"
        return formfield