求助有关nested嵌套obj的mapping设置问题

es版本5.6.3这个结构的文档doc：

{

    "total": 5,

    "online": [

        {

            "sid": "1101006",

            "total": 0

        },

        {

            "sid": "1101001",

            "total": 2,

            "platform": {

                "Windows_2144": 1,

                "Android_2144szsgnew": 1

            }

        },

        {

            "sid": "1101003",

            "total": 1,

            "platform": {

                "Android_2144szsgnew": 1

            }

        },

        {

            "sid": "1101004",

            "total": 2,

            "platform": {

                "Android_2144szsg": 2

            }

        },

        {

            "sid": "1101002",

            "total": 0

        },

        {

            "sid": "1101005",

            "total": 0

        }

    ],

    "_t": "2018-11-26 00:00:00",

    "groupid": "1101001"

}

 我在设置映射的时候"online"字段需要设置成nested，然后子结构里面有个"platform"是个object，这个结构里面的字段不是固定的，有可能动态增加，所以我想不把它写死，映射我设计成下面的结构，但是再插入数据的时候报错

{

    "error": {

        "root_cause": [

            {

                "type": "illegal_argument_exception",

                "reason": "object mapping [online] can't be changed from nested to non-nested"

            }

        ],

        "type": "illegal_argument_exception",

        "reason": "object mapping [online] can't be changed from nested to non-nested"

    },

    "status": 400

}

这个报错可以确定是"platform"字段的问题，我想问下该怎么改映射，才能插入这个结构的数据？
 
当前mapping：

{

    "settings": {

        "number_of_shards": 5,

        "number_of_replicas": 0

    },

    "mappings": {

        "doc": {

            "dynamic": true,

            "properties": {

                "total": {

                    "type": "integer"

                },

                "online": {

                    "type": "nested",

                    "properties": {

                        "sid": {

                            "type": "keyword"

                        },

                        "total": {

                            "type": "integer"

                        },

                        "platform": {

                            "dynamic": true,

                            "type": "object"

                        }

                    }

                },

                "_t": {

                    "type": "date",

                    "format": "YYYY-MM-dd HH:mm:ss"

                },

                "groupid": {

                    "type": "keyword"

                }

            }

        }

    }

}