post 仅在调用更新时保存信号

Question

你好，我有两个模型和一个 post_save 信号，但我的信号只有在更新对象时有效，而在我创建新对象时它不起作用

我的代码：

class Attendance(models.Model):
    classroom = models.ForeignKey('Class', on_delete=models.CASCADE)
    student = models.ForeignKey(User, on_delete=models.CASCADE)
    is_present = models.BooleanField(default=False)
    present_time = models.DateTimeField(null=True, blank=True)

    def __str__(self):
        return self.student.username


class Class(models.Model):
    course = models.ForeignKey(Course, on_delete=models.CASCADE)
    students = models.ManyToManyField(User)
    quantity = models.PositiveSmallIntegerField()
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()

    class Meta:
        unique_together = ['quantity', 'course']


    def __str__(self):
        return f'{self.quantity} / {self.course}'


@receiver(post_save, sender=Class)
def create_attendance(sender, instance, **kwargs):
    for student in instance.students.all():
        try:
            Attendance.objects.get(classroom=instance, student=student)
        except Attendance.DoesNotExist:
            Attendance.objects.create(classroom=instance, student=student)

Answer 1

在您创建实例的那一刻， instance.students 尚未填充（尚未），因此 empty，这意味着 for student in instance.students.all() 将枚举空集合，因此根本不会枚举 .

这是因为它首先需要创建一个带有主键的记录，然后将其用于 junction table，这是（许多) 使用信号通常不是一个好主意的原因。

使用 m2m_changed signal [Django-doc] 可能会更好，因此可以这样实现：

@receiver(m2m_changed, sender=Class.students.through)
def create_attendance(sender, instance, action, pk_set=None, **kwargs):
    if action == 'post_add':
        for <b>pk in pk_set</b>:
            Attendance.objects.get_or_create(classroom=instance, <b>student_id=pk</b>)

但是造型看起来有点奇怪。为什么不让 Attendance 成为你 ManyToManyField 的 through model [Django-doc]？这样 Django 将创建 Attendance 记录，然后您可以相应地更新这些记录。

Answer 2

post_save 信号中有一个 created 参数，当创建实例时为 True，当创建实例时为 False未创建（更新）。

@receiver(post_save, sender=Class)
def create_attendance(sender, instance, created, **kwargs):
    if not created:  
        for student in instance.students.all():
            try:
                Attendance.objects.get(classroom=instance, student=student)
            except Attendance.DoesNotExist:
                Attendance.objects.create(classroom=instance, student=student)

post 仅在调用更新时保存信号

post save signal only when update called

django

django-signals

django-models