删除关系时，Django Python m2m_change 不起作用

Posted 2023-05-07

【中文标题】删除关系时，Django Python m2m_change 不起作用

【英文标题】：Django Python m2m_change does not work when removing the relation

【发布时间】：2013-07-05 09:17:31

【问题描述】：

我正在编写一个与课程相关的项目。但是，当我使用 Django admin 添加 CourseSession 时，save() 不起作用。但是当我编辑它然后保存时，它会起作用。请帮我。以下是课程 CourseSession 和 Course。我这里做的是在添加相关课程会话时自动更新每门课程的讲师。（更新：我使用了m2m_change功能，但删除课程会话时不起作用）

def course_session_instructor_changed(sender, instance, action, **kwargs):
superCourse = instance.course
superCourse.instructors.clear()


course_session_set = superCourse.course_session.all()
for each_course_session in course_session_set:
    # add instructor
    if action=="post_add":
        instructors = each_course_session.instructors.all()
        for instructor in instructors:
            if not instructor in superCourse.instructors.all():
                superCourse.instructors.add(instructor)

    # remove instructor
    elif action=="pre_remove" :
        if not each_course_session == instance:
            instructors = each_course_session.instructors.all()
            for instructor in instructors:
                if not instructor in superCourse.instructors.all():
                    superCourse.instructors.add(instructor)

superCourse.save()

m2m_changed.connect(course_session_instructor_changed,     sender=CourseSession.instructors.through)

class CourseSession(models.Model):
    course = models.ForeignKey('Course', related_name='course_session')

    instructors = models.ManyToManyField(User, related_name = 'instructor_course_session')
    enrollment = models.ManyToManyField(User, related_name = 'course_enrollment')    
    start = models.DateField()
    # Weeks of duration
    duration = models.IntegerField()
    # capacity of the session
    max_cap = models.IntegerField()

    questionSet = models.ManyToManyField(QuestionSet, blank=True, null=True, related_name='session_questionSet')

    class Meta:
        verbose_name = _('Session')
        verbose_name_plural = _('Sessions')
        get_latest_by = "start"

    def __unicode__(self):
        return unicode(self.instructors.all())+unicode(self.course)+unicode(self.start)

    def is_started(self):
        return date.today()> self.start

    def is_expired(self):
        length = timedelta(days = self.duration*7)
        return self.start+length< date.today()

    def get_enrollment(self):
        return self.enrollment.count()




**class Course(models.Model):
    name = models.CharField(_('Course Name'),max_length=256)
    # Simple Introduction
    brief_intro = models.CharField(_('Brief Intro'),max_length=1024)
    intro = models.TextField()
    learning_obj = models.TextField()
    creator = models.ForeignKey(User, related_name = 'course_creator')
    created = models.DateTimeField(auto_now_add=True)    
    cover = models.ImageField(upload_to = 'course/covers/')
    institute = models.ForeignKey('Institute', related_name='institute_courses')
    workload = models.IntegerField()
    assignments = models.IntegerField()
    exams = models.IntegerField()   
    knowledge_tree_root = models.ForeignKey(Topic, related_name='knowledge_tree_root')
    instructors = models.ManyToManyField(User, related_name='courses', null=True, blank=True)

    #tree_root = models.ForeignKey('Topic')

    class Meta:
        verbose_name = _('Course')
        verbose_name_plural = _('Courses')

    def __unicode__(self):
        return self.name

【参考方案1】：

第一次无法使用，因为在保存一次实例之前无法设置多对多字段（因为它们保存在单独的表中，并且必须有一个 ID 才能链接到）。所以管理员在保存之前不会设置值。

您可能想要使用m2m_changed signal，而不是覆盖保存。

【讨论】：

我写了一个 m2m_change 函数。它确实在第一次工作。但是，当我删除课程会话时，它不会被触发。你能告诉我为什么吗？