class Notification(models.Model):
  """一个简化过的Notification类，拥有三个字段：

  - `user_id`: 消息所有人的用户ID
  - `has_readed`: 表示消息是否已读
  """

  user_id = models.IntegerField(db_index=True)
  has_readed = models.BooleanField(default=False)

# 获取ID为3074的用户的未读消息数
Notification.objects.filter(user_id=3074, has_readed=False).count()

class UserNotificationsCount(models.Model):
  """这个Model保存着每一个用户的未读消息数目"""

  user_id = models.IntegerField(primary_key=True)
  unread_count = models.IntegerField(default=0)

  def __str__(self):
    return '<UserNotificationsCount %s: %s>' % (self.user_id, self.unread_count)

from django.db.models.signals import post_save, post_delete

def incr_notifications_counter(sender, instance, created, **kwargs):
  # 只有当这个instance是新创建，而且has_readed是默认的false才更新
  if not (created and not instance.has_readed):
    return

  # 调用 update_unread_count 方法来更新计数器 +1
  NotificationController(instance.user_id).update_unread_count(1)

# 监听Notification Model的post_save信号
post_save.connect(incr_notifications_counter, sender=Notification)

def decr_notifications_counter(sender, instance, **kwargs):
  # 当删除的消息还没有被读过时，计数器 -1
  if not instance.has_readed:
    NotificationController(instance.user_id).update_unread_count(-1)

post_delete.connect(decr_notifications_counter, sender=Notification)

class NotificationController(object):

  ... ...

  def mark_as_readed(self, notification_id):
    notification = Notification.objects.get(pk=notification_id)
    # 没有必要重复标记一个已经读过的通知
    if notication.has_readed:
      return

    notification.has_readed = True
    notification.save()
    # 在这里更新我们的计数器，嗯，我感觉好极了
    self.update_unread_count(-1)

# 因为两个并发的请求，假设这两个方法几乎同时被调用
NotificationController(user_id).mark_as_readed(100)
NotificationController(user_id).mark_as_readed(100)

from django.db import transaction

class NotificationController(object):

  ... ...

  def mark_as_readed(self, notification_id):
    # 手动让select for update和update语句发生在一个完整的事务里面
    with transaction.commit_on_success():
      # 使用select_for_update来保证并发请求同时只有一个请求在处理，其他的请求
      # 等待锁释放
      notification = Notification.objects.select_for_update().get(pk=notification_id)
      # 没有必要重复标记一个已经读过的通知
      if notication.has_readed:
        return

      notification.has_readed = True
      notification.save()
      # 在这里更新我们的计数器，嗯，我感觉好极了
      self.update_unread_count(-1)

def mark_as_readed(self, notification_id):
    affected_rows = Notification.objects.filter(pk=notification_id, has_readed=False)                      .update(has_readed=True)
    # affected_rows将会返回update语句修改的条目数
    self.update_unread_count(affected_rows)

from django.db.models import F

def update_unread_count(self, count)
  # 使用Update语句来更新我们的计数器
  UserNotificationsCount.objects.filter(pk=self.user_id)                 .update(unread_count=F('unread_count') + count)

RK_NOTIFICATIONS_COUNTER = 'ss_pending_counter_changes'

def update_unread_count(self, count):
  """修改过的update_unread_count方法"""
  redisdb.zincrby(RK_NOTIFICATIONS_COUNTER, str(self.user_id), count)

# 同时我们也需要修改获取用户未读消息数方法，使其获取redis中那些没有被回写
# 到数据库的缓冲区数据。在这里代码就省略了

# File: management/commands/notification_update_counter.py

# -*- coding: utf-8 -*-
from django.core.management.base import BaseCommand
from django.db.models import F

# Fix import prob
from notification.models import UserNotificationsCount
from notification.utils import RK_NOTIFICATIONS_COUNTER
from base_redis import redisdb

import logging
logger = logging.getLogger('stdout')


class Command(BaseCommand):
  help = 'Update UserNotificationsCounter objects, Write changes from redis to database'

  def handle(self, *args, **options):
    # 首先，通过 zrange 命令来获取缓冲区所有修改过的用户ID
    for user_id in redisdb.zrange(RK_NOTIFICATIONS_COUNTER, 0, -1):
      # 这里值得注意，为了保证操作的原子性，我们使用了redisdb的pipeline
      pipe = redisdb.pipeline()
      pipe.zscore(RK_NOTIFICATIONS_COUNTER, user_id)
      pipe.zrem(RK_NOTIFICATIONS_COUNTER, user_id)
      count, _ = pipe.execute()
      count = int(count)
      if not count:
        continue

      logger.info('Updating unread count user %s: count %s' % (user_id, count))
      UserNotificationsCount.objects.filter(pk=obj.pk)                     .update(unread_count=F('unread_count') + count)