Django模型自定义保存与ManyToManyField问题

Question

我知道这个问题已被多次发布,但我仍然找不到这个问题的明确答案.所以,我走了:

class Invoice(models.Model):
    program = models.ForeignKey(Program)
    customer = models.ForeignKey(Customer, related_name='invoices')
    participants = models.ManyToManyField(Participant, related_name='participants_set')
    subtotal = models.DecimalField(max_digits=10, decimal_places=2, default='0.00', blank=True, null=False)
    pst = models.DecimalField("PST", max_digits=10, decimal_places=2, default='0.00', blank=True, null=False)
    gst = models.DecimalField("GST", max_digits=10, decimal_places=2, default='0.00', blank=True, null=False)
    total = models.DecimalField(max_digits=10, decimal_places=2, default='0.00', blank=True, null=False)

    def save(self, **kwargs):
        super(Invoice, self).save(**kwargs)
        items = self.participants.count()
        subtotal = Decimal(self.program.fee) * items
        pst = self.program.is_pst and Decimal(PST)*subtotal or Decimal('0.00')
        gst = self.program.is_gst and Decimal(GST)*subtotal or Decimal('0.00')
        total = (subtotal + pst) + gst
        self.subtotal = subtotal
        self.pst = pst
        self.gst = gst
        self.total = total
        super(Invoice, self).save(**kwargs)

一切正常,除了self.participants.count()不起作用.知道可能是什么问题.任何帮助非常感谢.

Answer 1

self.participants.all().count()