django如何计算商品数量*购物车中每个商品的数量

# models.py
from django.db import models

class Product(models.Model):
    name = models.CharField(max_length=100)
    # 其他商品相关字段...

class Cart(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.PositiveIntegerField()

    def calculate_total_quantity(self):
        total_quantity = 0
        cart_items = Cart.objects.filter(user=self.user)
        for item in cart_items:
            total_quantity += item.quantity
        return total_quantity

# views.py
from django.shortcuts import render
from .models import Cart

def cart_view(request):
    user = request.user
    cart = Cart.objects.filter(user=user)
    total_quantity = cart.calculate_total_quantity()
    # 其他处理逻辑...
    return render(request, 'cart.html', {'total_quantity': total_quantity})