# How to convert a nested field type?
**URL:** https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025
**Category:** Logstash
**Created:** [April 1, 2016, 6:22am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025 "2016-04-01T06:22:42Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 1, 2016, 6:22am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/1 "2016-04-01T06:22:42Z")
I find `mutate` filter can convert a field type, but how to convert a nested one?
I've tried:
```auto
mutate {
convert => {"logdetail.params" => "string"}
}
```
And
```auto
mutate {
convert => {"[logdetail][params]" => "string"}
}
```
But failed.
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 1, 2016, 6:24am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/2 "2016-04-01T06:24:57Z")
The latter should work. What does the message look like? Please use a `stdout { codec => rubydebug }` output so we get the raw data.
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 1, 2016, 6:26am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/3 "2016-04-01T06:26:34Z")
```auto
{
"@timestamp" => 2016-04-01T06:17:16.494Z,
"host" => "shifudao",
"type" => "rtds",
"loglevel" => "WARN",
"classname" => "hawkeyes.rtds.gate.protocol.frametype.SamplerReportData",
"logdetail" => {
"message" => "设备00004650数据上报帧里的sid为空，可能未发过心跳，丢弃！",
"action" => "fireEquipmentDataArrived(String writeHandlerID, String sid, Vertx vertx, List result)",
"sid" => nil,
"params" => {
"writeHandlerID" => "b6728301-653e-488c-91b8-fc2aa52c1312",
"sid" => nil,
"result" => [
[0] {
"eId" => "00004650",
"date" => 963328184494,
"items" => [
[0] [
[0] 1,
[1] [
[0] 74,
[1] 0,
[2] 0,
[3] 0
]
],
[1] [
[0] 2,
[1] [
[0] -119,
[1] 0,
[2] 0,
[3] 0
]
],
[2] [
[0] 3,
[1] [
[0] 12,
[1] 0,
[2] 0,
[3] 0
]
],
[3] [
[0] 4,
[1] [
[0] 12,
[1] 0,
[2] 0,
[3] 0
]
],
[4] [
[0] 5,
[1] [
[0] 110,
[1] 5,
[2] 0,
[3] 0
]
],
[5] [
[0] 6,
[1] [
[0] 61,
[1] 5,
[2] 0,
[3] 0
]
],
[6] [
[0] 7,
[1] [
[0] -18,
[1] 5,
[2] 0,
[3] 0
]
],
[7] [
[0] 8,
[1] [
[0] 3,
[1] 8,
[2] 0,
[3] 0
]
],
[8] [
[0] 9,
[1] [
[0] 0,
[1] 0,
[2] 0,
[3] 0
]
]
]
}
]
}
},
"@version" => "1"
}
```
`logdetail.params` is a nested json object, and I want to convert it to string.
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 1, 2016, 7:32am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/4 "2016-04-01T07:32:13Z")
Oh. Such objects can't be converted to a string (see below). Perhaps the [json\_encode filter](https://www.elastic.co/guide/en/logstash/current/plugins-filters-json_encode.html) would be useful?
>
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 1, 2016, 7:32am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/5 "2016-04-01T07:32:58Z")
Oh, I read the official doc carefully, and I find this reason:
```auto
If the field is a hash, no action will be taken.
```
So this field is a hash. But how to I perform a `toString()` action?
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 1, 2016, 12:40pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/6 "2016-04-01T12:40:01Z")
But `json_encode` convert a string to json. I want to convert a nested json to string. How to?
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 1, 2016, 1:47pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/7 "2016-04-01T13:47:24Z")
No, you have a field that happens to be nested that you for some reason want to transform to a string representation. The json\_encode filter serializes a field to JSON. JSON is a string.
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 1, 2016, 2:24pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/8 "2016-04-01T14:24:45Z")
Great! `json_encode` works for me now.
And is there any built-in filter alternative ? Maybe `ruby` could also work, but I'm not good at not ruby. I don't know how to use `ruby` to implement.
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 2, 2016, 2:35pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/9 "2016-04-02T14:35:40Z")
> And is there any built-in filter alternative ?
Alternative that does what?
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 2, 2016, 5:53pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/10 "2016-04-02T17:53:20Z")
Do the same with json\_encode, just like my title. But it's a built-in filter. Maybe the ruby filter could do the same thing, but I'm not good at ruby, is there any examples?
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 2, 2016, 5:57pm UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/11 "2016-04-02T17:57:07Z")
So what you're really asking for is JSON serialization with a core plugin? That should indeed be possible with a ruby filter but I don't have any example at hand, except of course the json\_encode source code.
---
### Author:  [@abcfy2](https://discuss.elastic.co/u/abcfy2)
#### Post date: [April 3, 2016, 2:42am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/12 "2016-04-03T02:42:01Z")
Thanks all the same.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [July 6, 2017, 5:04am UTC](https://discuss.elastic.co/t/how-to-convert-a-nested-field-type/46025/13 "2017-07-06T05:04:12Z")