class Notification(models.Model):
  """一個簡化過的Notification類，擁有三個字段：

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

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

# 獲取ID為3074的用戶的未讀消息數(shù)
Notification.objects.filter(user_id=3074, has_readed=False).count()

class UserNotificationsCount(models.Model):
  """這個Model保存著每一個用戶的未讀消息數(shù)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):
  # 只有當(dāng)這個instance是新創(chuàng)建，而且has_readed是默認(rèn)的false才更新
  if not (created and not instance.has_readed):
    return

  # 調(diào)用 update_unread_count 方法來更新計數(shù)器 +1
  NotificationController(instance.user_id).update_unread_count(1)

# 監(jiān)聽Notification Model的post_save信號
post_save.connect(incr_notifications_counter, sender=Notification)

def decr_notifications_counter(sender, instance, **kwargs):
  # 當(dāng)刪除的消息還沒有被讀過時，計數(shù)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)
    # 沒有必要重復(fù)標(biāo)記一個已經(jīng)讀過的通知
    if notication.has_readed:
      return

    notification.has_readed = True
    notification.save()
    # 在這里更新我們的計數(shù)器，嗯，我感覺好極了
    self.update_unread_count(-1)

# 因為兩個并發(fā)的請求，假設(shè)這兩個方法幾乎同時被調(diào)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):
    # 手動讓select for update和update語句發(fā)生在一個完整的事務(wù)里面
    with transaction.commit_on_success():
      # 使用select_for_update來保證并發(fā)請求同時只有一個請求在處理，其他的請求
      # 等待鎖釋放
      notification = Notification.objects.select_for_update().get(pk=notification_id)
      # 沒有必要重復(fù)標(biāo)記一個已經(jīng)讀過的通知
      if notication.has_readed:
        return

      notification.has_readed = True
      notification.save()
      # 在這里更新我們的計數(shù)器，嗯，我感覺好極了
      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將會返回update語句修改的條目數(shù)
    self.update_unread_count(affected_rows)

from django.db.models import F

def update_unread_count(self, count)
  # 使用Update語句來更新我們的計數(shù)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):
  """修改過的update_unread_count方法"""
  redisdb.zincrby(RK_NOTIFICATIONS_COUNTER, str(self.user_id), count)

# 同時我們也需要修改獲取用戶未讀消息數(shù)方法，使其獲取redis中那些沒有被回寫
# 到數(shù)據(jù)庫的緩沖區(qū)數(shù)據(jù)。在這里代碼就省略了

# 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，通過 zrange 命令來獲取緩沖區(qū)所有修改過的用戶ID
    for user_id in redisdb.zrange(RK_NOTIFICATIONS_COUNTER, 0, -1):
      # 這里值得注意，為了保證操作的原子性，我們使用了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)