mysite/feed/models.py

 

#!/usr/bin/python # -*- coding: utf-8 -*- from django.db import models from django.db.models import F     class Account(models.Model): """ 账户"""     balance = models.DecimalField('余额', max_digits=19, decimal_places=4)     version = models.IntegerField('版本号')     created = models.DateTimeField('创建时间', auto_now_add=True)     updated = models.DateTimeField('最后修改时间', auto_now=True) def add(self, amount): """ 增加金额"""         self.balance += amount         self.save()         self = self.__class__.objects.get(pk=self.id)         TradeRecord(             account=self,             amount=amount,             balance=self.balance,         ).save() return self.balance def f_add(self, amount): """ F函数"""         self.balance = F('balance') + amount         self.save()         self = self.__class__.objects.get(pk=self.id)         TradeRecord(             account=self,             amount=amount,             balance=self.balance,         ).save() return self.balance def lock_add(self, amount): """ 乐观锁"""         self.balance += amount         self.version += 1         new_self = self.__class__.objects.get(pk=self.id) print self.version, new_self.version if self.version > new_self.version:             self.save()             TradeRecord(                 account=self,                 amount=amount,                 balance=self.balance,             ).save() return self.balance return False class TradeRecord(models.Model): """ 交易明细"""     account = models.ForeignKey(Account)     amount = models.DecimalField('交易金额', max_digits=19, decimal_places=4)     balance = models.DecimalField('交易后余额', max_digits=19, decimal_places=4)     created = models.DateTimeField('创建时间', auto_now_add=True)     updated = models.DateTimeField('最后修改时间', auto_now=True)

终端1：

 

>>> from mysite.feed.models import * >>> account = Account.objects.get(pk=1) >>> account.balance Decimal("9.0000") >>> account.add(1) Decimal("10.0000") >>> account.balance Decimal("10.0000") >>>

终端2：

 

>>> from mysite.feed.models import * >>> account = Account.objects.get(pk=1) >>> account.balance Decimal("10.0000") >>> account.add(1) Decimal("11.0000") >>> account.balance Decimal("11.0000") >>>

终端1：

 

>>> account.balance Decimal("10.0000") >>> account.add(1) Decimal("11.0000") >>> account.balance Decimal("11.0000") >>>

初始值为9，在终端1进行了两次加1，终端2进行了一次加1操作，期望值为12，实际值为11。因为在终端1进行第二次加1操作时，余额已经读出为10.

使用F函数

终端2：

 

>>> from django.db.models import F >>> account.balance Decimal("11.0000") >>> account.balance = F('balance') + 1 >>> account.save() >>> account.balance 
     
       >>> account = Account.objects.get(pk=1) >>> account.balance Decimal("12.0000") >>>
     

终端1：

 

>>> from django.db.models import F >>> account.balance Decimal("11.0000") >>> account.balance = F('balance') + 1 >>> account.save() >>> account.balance 
     
       >>> account = Account.objects.get(pk=1) >>> account.balance Decimal("13.0000") >>>
     

在终端1执行时，虽然已经读出balance的值为11，但是加1后的结果仍然是13。可以看出使用F函数进行累加，在执行Save时，始终会去取数据库中最新的值，而不是已经取出的值。所以我们可以用F函数改写add函数，来保证并发执行时数据的正确性。

我们可以看到Django ORM在处理这两种累加在实际执行的SQL语句上的差异。

from django.db.models import F from mysite.feed.models import * account = Account.objects.get(pk=1) account 
     
       account.balance Decimal("0.0000") from django.db import connection connection.queries
     

[{'time': '0.000', 'sql': u'SELECT `feed_account`.`id`, `feed_account`.`balance`, `feed_account`.`version`, `feed_account`.`created`, `feed_account`.`updated` FROM `feed_account` WHERE `feed_account`.`id` = 1 '}]

 

account.balance += 1 connection.queries

[{'time': '0.000', 'sql': u'SELECT `feed_account`.`id`, `feed_account`.`balance`, `feed_account`.`version`, `feed_account`.`created`, `feed_account`.`updated` FROM `feed_account` WHERE `feed_account`.`id` = 1 '}]

account.save() connection.queries

[{'time': '0.000', 'sql': u'SELECT `feed_account`.`id`, `feed_account`.`balance`, `feed_account`.`version`, `feed_account`.`created`, `feed_account`.`updated` FROM `feed_account` WHERE `feed_account`.`id` = 1 '}, {'time': '0.000', 'sql': u'SELECT (1) AS `a` FROM `feed_account` WHERE `feed_account`.`id` = 1 LIMIT 1'}, {'time': '0.000', 'sql': u'UPDATE `feed_account` SET`balance` = 1.0000, `version` = 1, `created` = 2011-06-10 14:20:36, `updated` = 2011-06-10 16:00:52 WHERE `feed_account`.`id` = 1 '}]

 

account.balance = F('balance') + 1 account.save() connection.queries

[{'time': '0.000', 'sql': u'SELECT `feed_account`.`id`, `feed_account`.`balance`, `feed_account`.`version`, `feed_account`.`created`, `feed_account`.`updated` FROM `feed_account` WHERE `feed_account`.`id` = 1 '}, {'time': '0.000', 'sql': u'SELECT (1) AS `a` FROM `feed_account` WHERE `feed_account`.`id` = 1 LIMIT 1'}, {'time': '0.000', 'sql': u'UPDATE `feed_account` SET `balance` = 1.0000, `version` = 1, `created` = 2011-06-10 14:20:36, `updated` = 2011-06-10 16:00:52 WHERE `feed_account`.`id` = 1 '}, {'time': '0.000', 'sql': u'SELECT (1) AS `a` FROM `feed_account` WHERE `feed_account`.`id` = 1 LIMIT 1'}, {'time': '0.000', 'sql': u'UPDATE `feed_account` SET `balance` = `feed_account`.`balance` + 1, `version` = 1, `created` = 2011-06-10 14:20:36, `updated` = 2011-06-10 16:02:21 WHERE `feed_account`.`id` = 1 '}]

 

参考资料：