Django QuerySet中的Sum计算列

鉴于以下情况 Contribution

class Contribution(models.Model):
    start_time = models.DateTimeField()
    end_time = models.DateTimeField(null=True)

SELECT SUM(end_time - start_time) AS total_duration FROM contribution;

我想了很多:

Contribution.objects.aggregate(total_duration=models.Sum( ??? ))

但我不知道如何代表 end_time - start_time