Django ORM and locking table

Question

My problem is as follows:

I have a car dealer A, and a db table named sold_cars. When a car is being sold I create entry in this table.

Table ha

Answer 1

I think this code snippet meets your need, assuming you are using MySQL. If not, you may need to tweak the syntax a little, but the idea should still work.

Source: Locking tables

class LockingManager(models.Manager):
    """ Add lock/unlock functionality to manager.

    Example::

        class Job(models.Model):

            manager = LockingManager()

            counter = models.IntegerField(null=True, default=0)

            @staticmethod
            def do_atomic_update(job_id)
                ''' Updates job integer, keeping it below 5 '''
                try:
                    # Ensure only one HTTP request can do this update at once.
                    Job.objects.lock()

                    job = Job.object.get(id=job_id)
                    # If we don't lock the tables two simultanous
                    # requests might both increase the counter
                    # going over 5
                    if job.counter < 5:
                        job.counter += 1                                        
                        job.save()

                finally:
                    Job.objects.unlock()


    """    

    def lock(self):
        """ Lock table. 

        Locks the object model table so that atomic update is possible.
        Simulatenous database access request pend until the lock is unlock()'ed.

        Note: If you need to lock multiple tables, you need to do lock them
        all in one SQL clause and this function is not enough. To avoid
        dead lock, all tables must be locked in the same order.

        See http://dev.mysql.com/doc/refman/5.0/en/lock-tables.html
        """
        cursor = connection.cursor()
        table = self.model._meta.db_table
        logger.debug("Locking table %s" % table)
        cursor.execute("LOCK TABLES %s WRITE" % table)
        row = cursor.fetchone()
        return row

    def unlock(self):
        """ Unlock the table. """
        cursor = connection.cursor()
        table = self.model._meta.db_table
        cursor.execute("UNLOCK TABLES")
        row = cursor.fetchone()
        return row