select_for_update() for Django < 1.4. Tested on Django 1.3.

django_select_for_update.py

"""
Adds `select_for_update()` method to Django QuerySet (Django < 1.4).

Usage:
class MyModel(models.Model):
    if not hasattr(models.Manager, 'select_for_update'):
        objects = ForUpdateManager()
        
    # model definition
"""
__author__ = "Renskiy <[email protected]>"
from functools import wraps
from django.conf import settings
from django.db import models
from django.db.models.query import QuerySet
from django.db.models.sql import Query


def for_update(as_sql):
    """
    Decorator: for_update

    Used to decorate `as_sql` method of Django SQLCompiler,
    adds " FOR UPDATE" to the end of result of decorated method.
    """

    @wraps(for_update)
    def _wrapper(compiler, *args, **kwargs):
        sql, params = as_sql(compiler, *args, **kwargs)
        if getattr(compiler.query, 'select_for_update', False):
            sql += ' FOR UPDATE'
        return sql, params

    return _wrapper


class ForUpdateQuery(Query):
    """
    Class: ForUpdateQuery
    
    Query which adds support of `select_for_update()`.
    """

    def __init__(self, *args, **kwargs):
        super(ForUpdateQuery, self).__init__(*args, **kwargs)
        self.select_for_update = False

    def get_compiler(self, *args, **kwargs):
        compiler = super(ForUpdateQuery, self).get_compiler(*args, **kwargs)
        as_sql = getattr(compiler.__class__, 'as_sql')
        if as_sql.__name__ != 'for_update':
            # decorate class method only if it's not decorated yet
            decorated_as_sql = for_update(as_sql)
            setattr(compiler.__class__, 'as_sql', decorated_as_sql)
        return compiler

    def clone(self, *args, **kwargs):
        kwargs.update(select_for_update=self.select_for_update)
        return super(ForUpdateQuery, self).clone(*args, **kwargs)


class ForUpdateQuerySet(QuerySet):
    """
    Class: ForUpdateQuerySet

    Special query class for replacing Django's standard one, adds select_for_update() method
    """

    def select_for_update(self):
        db_engine = settings.DATABASES.get(self.db, {}).get('ENGINE', '').lower()
        if 'sqlite' in db_engine:  # sqlite doesn't support "FOR UPDATE"
            return self
        query_set = self._clone()
        query_set.query._for_write = True
        query_set.query.select_for_update = True
        return query_set


class ForUpdateManager(models.Manager):
    """
    Class: ForUpdateManager

    Special model manager for replacing Django's standard one, adds select_for_update() method
    """

    def get_query_set(self):
        query = ForUpdateQuery(self.model)
        return ForUpdateQuerySet(self.model, using=self._db, query=query)

    def select_for_update(self):
        return self.get_query_set().select_for_update()