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m2m 字段上的 post_save 信号

发布于 2024-07-08 03:33:08 字数 115 浏览 8 评论 0原文

我有一个非常通用的文章模型，与标签模型有 m2m 关系。我想记录每个标签的使用情况，我认为最好的方法是对标签模型上的计数字段进行非规范化，并在每次保存文章时更新它。我怎样才能做到这一点，或者也许有更好的方法？

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滥情稳全场 2024-07-15 03:33:08

这是 Django 1.2 中的新功能：
http://docs.djangoproject.com/en/dev/ref /信号/#m2m-更改

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想你只要分分秒秒 2024-07-15 03:33:08

您可以通过创建 M2M 关系的中间模型，并将其用作 post_save 和 post_delete 信号的挂钩，以更新 中的非规范化列文章表。

例如，我对 soclone 中收藏的问题计数执行此操作，其中用户与问题有M2M关系：

from django.contrib.auth.models import User
from django.db import connection, models, transaction
from django.db.models.signals import post_delete, post_save

class Question(models.Model):
    # ...
    favourite_count = models.PositiveIntegerField(default=0)

class FavouriteQuestion(models.Model):
    question = models.ForeignKey(Question)
    user     = models.ForeignKey(User)

def update_question_favourite_count(instance, **kwargs):
    """
    Updates the favourite count for the Question related to the given
    FavouriteQuestion.
    """
    if kwargs.get('raw', False):
        return
    cursor = connection.cursor()
    cursor.execute(
        'UPDATE soclone_question SET favourite_count = ('
            'SELECT COUNT(*) from soclone_favouritequestion '
            'WHERE soclone_favouritequestion.question_id = soclone_question.id'
        ') '
        'WHERE id = %s', [instance.question_id])
    transaction.commit_unless_managed()

post_save.connect(update_question_favourite_count, sender=FavouriteQuestion)
post_delete.connect(update_question_favourite_count, sender=FavouriteQuestion)

# Very, very naughty
User.add_to_class('favourite_questions',
                  models.ManyToManyField(Question, through=FavouriteQuestion,
                                         related_name='favourited_by'))

django开发者邮件列表上有一些关于实现一种以声明方式声明非规范化的方法以避免编写像上面这样的代码：

You can do this by creating an intermediate model for the M2M relationship and use it as your hook for the post_save and post_delete signals to update the denormalised column in the Article table.

For example, I do this for favourited Question counts in soclone, where Users have a M2M relationship with Questions:

from django.contrib.auth.models import User
from django.db import connection, models, transaction
from django.db.models.signals import post_delete, post_save

class Question(models.Model):
    # ...
    favourite_count = models.PositiveIntegerField(default=0)

class FavouriteQuestion(models.Model):
    question = models.ForeignKey(Question)
    user     = models.ForeignKey(User)

def update_question_favourite_count(instance, **kwargs):
    """
    Updates the favourite count for the Question related to the given
    FavouriteQuestion.
    """
    if kwargs.get('raw', False):
        return
    cursor = connection.cursor()
    cursor.execute(
        'UPDATE soclone_question SET favourite_count = ('
            'SELECT COUNT(*) from soclone_favouritequestion '
            'WHERE soclone_favouritequestion.question_id = soclone_question.id'
        ') '
        'WHERE id = %s', [instance.question_id])
    transaction.commit_unless_managed()

post_save.connect(update_question_favourite_count, sender=FavouriteQuestion)
post_delete.connect(update_question_favourite_count, sender=FavouriteQuestion)

# Very, very naughty
User.add_to_class('favourite_questions',
                  models.ManyToManyField(Question, through=FavouriteQuestion,
                                         related_name='favourited_by'))

There's been a bit of discussion on the django-developers mailing list about implementing a means of declaratively declaring denormalisations to avoid having to write code like the above:

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