将 django 中的两个字段相乘

Question

我有两种不同的型号

class Part(models.Model):
    partno = models.CharField(max_length=50)   
    price = models.PositiveIntegerField(default= 0)
    tax = models.PositiveIntegerField(default= 0)
    created_date = models.DateField(auto_now_add=True)

class PurchaseOrder(models.Model):

    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    amount = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

我想将 Part 的price与po_quantity ，结果显示在amount中。 例如

金额 = 价格 * po_quantity

有人可以帮我解决这个问题吗？

Answer 1

由于amount完全取决于price和po_quantity ，因此您最好为此使用属性。 否则你会引入数据重复。 因此，您可以将其实现为：

class PurchaseOrder(models.Model):
    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

    @property
    def amount(self):
        return self.po_quantity * self.part.price

如果您需要在过滤时使用amount ，您可以使用.annotate(…) [Django-doc]代替，并在Manager中定义它：

from django.db.models import F

class PurchaseOrderManager(models.Manager):
    
    def get_queryset(self, *args, **kwargs):
        return super().get_queryset(*args, **kwargs).annotate(
            amount=F('po_quantity') * F('part__price')
        )


class PurchaseOrder(models.Model):
    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

    objects = PurchaseOrderManager()