删除关系时，Django Python m2m\u change不起作用

Question

我正在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。但当我编辑它，然后保存，它会工作。请帮帮我。以下是班级课程和课程。我在这里做的是在添加相关课程时自动更新每门课程的讲师。（更新：我使用了m2m\U更改功能，但在删除课程时它不起作用）

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

Answer 1

它第一次不会工作，因为在实例保存一次之前不能设置多对多字段（因为它们保存在单独的表中，并且必须有一个要链接的ID）。所以管理员直到保存之后才设置值。你知道吗

您可能希望使用^{} signal，而不是重写save。你知道吗