Django:如何在同一模型中用另外两个字段的值自动填充一个字段

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

class MyModel(models.Model):
    field1 = models.CharField(max_length=100)
    field2 = models.CharField(max_length=100)
    field3 = models.CharField(max_length=100, blank=True)

@receiver(pre_save, sender=MyModel)
def fill_field3(sender, instance, *args, **kwargs):
    if not instance.field3:  # 只有当field3为空时才自动填充
        instance.field3 = instance.field1 + instance.field2