如何在django中的另一个模型保存方法中更新模型实例？

Question

I have a master model which creates alphanumeric automatically for different types of Vouchers for different companies. How do I update the master model. The models:

class VoucherTypeMaster(models.Model):
    code = models.CharField(max_length=12,null=True,blank=True)
    description = models.CharField(max_length=30,null=True,blank=True)
    last_number = models.IntegerField(null=True,blank=True)
    company = models.ForeignKey(Company,
                                   related_name='voucher_master_company')
    class Meta:
        unique_together = ('code','company')

class Voucher(models.Model):
    type = models.ForeignKey(VoucherTypeMaster)
    date = models.DateField(default=datetime.datetime.now().date())
    company = models.ForeignKey(Company,
                                  related_name='voucher_company')
    number = models.CharField(max_length=20,null=True,blank=True)
    narration = models.CharField(max_length=30,null=True,blank=True)
    amount = models.DecimalField(decimal_places=2,max_digits=9)

    # class Meta:
        # unique_together = ('company','number','date')

    def __unicode__(self):
        return '%s - %s' %(self.number,self.narration)

    def save(self, *args, **kwargs):
        try:
            voucher_type = VoucherTypeMaster.objects.get(
                company=self.company,
                code=self.type.code
                )
            voucher_type.last_number += 1
            voucher_type.save()
            self.number = voucher_type.last_number
#            self.type.save() # throws exception
        except Exception,e:
            print e

        super(Voucher, self).save(*args, **kwargs)

If I uncomment self.type.save() Traceback got:

Manager isn't accessible via VoucherTypeMaster instances

How to update the VoucherTypeMaster model with the next value? Using django 1.6.5, linux

Answer 1

Overriding the save method on Voucher model and passing the VoucherTypeMaster and not its instance solved the problem: Increment the last_number, if the self.id is None

def save(self, *args, **kwargs):
    try:
        voucher_type = VoucherTypeMaster.objects.get(
            company=self.company,
            code=self.type.code
            )
        if self.id is None:
            voucher_type.last_number = voucher_type.last_number+1
            self.type = voucher_type
            voucher_type.save()
    except Exception,e:
        print e
    super(Voucher, self).save(*args, **kwargs)