NOT NULL约束失败：stop_watch_activity.activity_type_id

Question

我已经审查了其他一些解决方案，但都无济于事，我正努力传递null，因此我可以一次处理我的代码，而不必让数据库包含所有需要的值。为什么我在列出activity_type_id时不能以null的身份登录null=True, blank=True？

User = get_user_model()
   
class ActivityType(models.Model):
    name = models.CharField(max_length=100)
    user = models.ForeignKey(User, on_delete=models.CASCADE)

    def __str__(self):
        return '{id} {name} {user_id}'.format(id=self.id, name=self.name, user_id=self.user_id)

class Activity(models.Model):
    name = models.CharField(max_length=100)
    owner = models.ForeignKey(User, on_delete=models.CASCADE)
    activity_type = models.ForeignKey(ActivityType, on_delete=models.CASCADE, null=True, blank=True)
    
    def __str__(self):
        return self.name

class ActivitySession(models.Model):
    activity = models.ForeignKey(Activity, on_delete=models.CASCADE)
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()