Django，如何设置duration字段为两个datetime字段之间的差异

from django.db import models

class TimeDurationField(models.DurationField):
    def from_db_value(self, value, expression, connection):
        if value is None:
            return None
        return datetime.timedelta(seconds=value.total_seconds())

    def to_python(self, value):
        if isinstance(value, datetime.timedelta):
            return value
        if value is None:
            return None
        return datetime.timedelta(seconds=value)

    def get_prep_value(self, value):
        if value is None:
            return None
        if isinstance(value, datetime.timedelta):
            return value.total_seconds()
        return value

from django.db import models

class MyModel(models.Model):
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()
    duration = TimeDurationField(editable=False)

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