修改购物车数量的Django视图/模型

from django.shortcuts import get_object_or_404
from django.http import JsonResponse

from .models import CartItem

def update_cart_item(request, cart_item_id):
    cart_item = get_object_or_404(CartItem, id=cart_item_id)
    
    if request.method == 'POST':
        quantity = request.POST.get('quantity')
        cart_item.quantity = quantity
        cart_item.save()
        
        return JsonResponse({'message': 'Cart item quantity updated successfully.'})
    
    return JsonResponse({'message': 'Invalid request method.'}, status=400)

from django.db import models

class CartItem(models.Model):
    product = models.ForeignKey('Product', on_delete=models.CASCADE)
    quantity = models.PositiveIntegerField(default=1)
    # 其他字段...

    def __str__(self):
        return self.product.name