Django:使用当前时间戳填充可空的DateTimeField？

from django.db import models

class MyModel(models.Model):
    timestamp = models.DateTimeField(null=True)

from django.db.models.signals import pre_save
from django.dispatch import receiver
from django.utils import timezone

@receiver(pre_save, sender=MyModel)
def update_timestamp(sender, instance, **kwargs):
    if not instance.timestamp:
        instance.timestamp = timezone.now()

from django.apps import AppConfig

class MyAppConfig(AppConfig):
    default_auto_field = 'django.db.models.BigAutoField'
    name = 'myapp'

    def ready(self):
        import myapp.signals

my_model = MyModel()
my_model.save()