DRF 嵌套序列化：读模型容易，写模型要显式设计

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嵌套 Serializer 很适合返回详情页所需的关联信息，但可写嵌套结构没有通用默认答案。

class OrderSerializer(serializers.ModelSerializer):
    items = OrderItemSerializer(many=True, read_only=True)

    class Meta:
        model = Order
        fields = ["id", "items", "total"]

当客户端要提交 items 时，先决定缺失项是保留、删除还是错误；再在事务内实现。不要因为方便而把整个关联对象开放为可写输入。