django在聚合期间对字段执行逻辑（而不是直接执行聚合（Sum（'somefield'）））

Code:

MyModel(models.Model):
    start_date = models.DateTimeField()
    end_date = models.DateTimeField()
    name = models.CharField()
    somenum = models.IntegerField()
MyModel(models.Model):
    start_date =