models.py

class Cartlist(models.
    user = models.ForeignKey(User, on_delete=models.
    created_at = models.DateField(auto_now_add=True)
    class Meta:
        ordering = ['id']
    def __str__(self):
        return self.user.username
class Items(models.Model):
    cart = models.ForeignKey(Cartlist, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField(default=1)
    order = models.BooleanField(default=False)
    created_at = models.DateField(auto_now_add=True)

    class Meta:
        ordering = ['id']

    def __str__(self):
        return '{}'.format(self.product)

Views.py

class AddCartView(generics.CreateAPIView):
    permission_classes = [IsAuthenticated]
    queryset =  Items.objects.all()
    serializer_class = AddCartSerializer

Serializer.py

class AddCartSerializer(serializers.ModelSerializer):
    class Meta:
        model = Items
        fields = ['product']

    def create(self, validated_data):
        user = self.context['request'].user 
        product = self.validated_data['product'] 
        if Cartlist.objects.filter(user=user):
            item = Items.objects.filter(cart__user=user,product=product)
            if item:
                item.quantity += 1
                item.save()
                return item
            else:
                item = Items.objects.create(cart=Cartlist.objects.get(user=user),**validated_data)
                item.save()
                return item
        
        else:
            cart = Cartlist.objects.create(user=user)
            cart.save()
            item = Items.objects.create(cart=cart,**validated_data)
            item.save()
            return item

Exception Value:'QuerySet' object has no attribute 'quantity'
如果产品已存在，则将数量字段增加1。默认值为1。如果用户将相同产品添加到其购物车列表中，则产品数量必须增加1