django表单和对象的元数据处理

class Job(models.Model):
    name = models.CharField(max_length=32, unique=True) #name of job
    start = models.TimeField() #start time of job
    end = models.TimeField() #end time of job
    days = models.ManyToManyField(Day) #week days the job should be started on

hash = models.CharField(max_length=.., Null=true, Blank=true)

def save(self):
    self.hash = calc(...)
    super(MyModel, self).save()