如何在Django模型中使用@property？

Posted 2021-04-05

我想在查询中添加一个子查询。然后在@property中创建了一个Transaction。在互联网上发现这就是我所需要的。但是我不完全了解它们是如何工作的。如何使用？

views.py（Query）

paymentsss = Transaction.objects.all().select_related('currency',
       'payment_source__payment_type',
     'deal__service__contractor',).

models.py

class PayerPaymentSource(models.Model):
    id = models.BigIntegerField(blank=True, null=False, primary_key=True)
    payer_id = models.BigIntegerField(blank=True, null=True)
    payment_type = models.ForeignKey(PaymentType, max_length=64, blank=True, null=True, on_delete=models.CASCADE)
    source_details = models.TextField(blank=True, null=True)  # This field type is a guess.

    class Meta:
        managed = False
        db_table = '"processing"."payer_payment_source"'


class Transaction(models.Model):
    id = models.BigIntegerField(blank=True, null=False, primary_key=True)
    currency = models.ForeignKey(Currency, null=True, on_delete=models.CASCADE)
    deal = models.ForeignKey(Deal, null=True, on_delete=models.CASCADE)
    # service_instance = models.ForeignKey(ServiceInstance, null=True, on_delete=models.CASCADE)
    payment_source = models.ForeignKey(PayerPaymentSource, null=True, on_delete=models.CASCADE)
    payment_date = models.DateTimeField(blank=True, null=True)
    amount = models.IntegerField(blank=True, null=True)
    status = models.CharField(max_length=255, blank=True, null=True)
    context = models.TextField(blank=True, null=True)

    @property
    def bank_card_details(self):
        return PayerPaymentSource.objects.filter(self.payment_source.source_details,
                                                 payment_type='bank_card_details')

    class Meta:
        managed = False
        db_table = '"processing"."transaction"'

UPD：print(payment.bank_card_details)有效，但它会创建许多类似的查询。如何解决？

答案

@ property装饰器只是调用内置在Python中的property（）函数的便捷方法。该函数返回一个特殊的descriptor object，它可以直接访问该方法的计算值。

例如您的观点

obj = Transaction.objects.get(pk=pk)
#now you can get the bank_card_details like this:
print(obj.bank_card_details)