Django模型，如何使两个可选字段，但当一个填充时，另一个也必须填充

from django.db import models

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

from django.core.exceptions import ValidationError

def validate_fields(value1, value2):
    if bool(value1) != bool(value2):
        raise ValidationError("field1 and field2 must both be filled or both be empty.")

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

@receiver(pre_save, sender=MyModel)
def validate_fields_before_save(sender, instance, **kwargs):
    validate_fields(instance.field1, instance.field2)