Django：从不同的表中获取值并计算字段

Question

我想从 'Product' 中获取 'weight' 并计算 'total_weight=weight*quantity'，如 html 所示

class Product(models.Model):
    code = models.CharField(max_length=64, unique=True)
    name = models.CharField(max_length=64)
    weight = models.DecimalField(max_digits=20, decimal_places=2)

class Invoice(models.Model):
   date = models.DateField(default=timezone.now)
   client = models.ForeignKey(Client,on_delete=models.CASCADE)

class InvoiceItem(models.Model):
    invoice = models.ForeignKey('Invoice', on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    price = models.DecimalField(max_digits=20, decimal_places=2)
    quantity = models.DecimalField(max_digits=20, decimal_places=2)

    def total(self):
        return round(self.price * self.quantity, 2)

还有html：

{% for product in object.invoiceitem_set.all %}
  <tr>
    <td>{{product.product}}</td>
    <td>{{product.price}}</td>
    <td>{{product.quantity}}</td>
    <td>{{product.total}}</td>
    <td>{{product.weight}}</td>
    <td>{{product.total_weight}}</td>
  </tr>

Answer 1

为 invoiceitem 模型添加总重量方法

   def total_weight(self):
        return  self.product.weight *  self.quantity