如何在django模型中减去两个DateTime字段，并将减去的值保留在同一模型中的另一个字段中？

from django.db import models

class MyModel(models.Model):
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()
    duration = models.DurationField(null=True, blank=True)

from django.db.models import F

class MyModel(models.Model):
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()
    duration = models.DurationField(null=True, blank=True)

    def save(self, *args, **kwargs):
        if self.start_time and self.end_time:
            self.duration = F('end_time') - F('start_time')
        super().save(*args, **kwargs)