# Filter mutate add\_field : select a value in an inner field

**URL:** https://discuss.elastic.co/t/filter-mutate-add-field-select-a-value-in-an-inner-field/124626
**Category:** Logstash
**Created:** [March 19, 2018, 6:42pm UTC](https://discuss.elastic.co/t/filter-mutate-add-field-select-a-value-in-an-inner-field/124626 "2018-03-19T18:42:00Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![Lemec\_Cinq](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/lemec_cinq/32/28949_2.png) [@Lemec\_Cinq](https://discuss.elastic.co/u/Lemec_Cinq)
#### Post date: [March 19, 2018, 6:42pm UTC](https://discuss.elastic.co/t/filter-mutate-add-field-select-a-value-in-an-inner-field/124626/1 "2018-03-19T18:42:00Z")

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I have a json input :

```
{"@version":"1","@timestamp":"2018-03-19T18:24:01.919Z","fields":"{\"numdocksavailable\":0,\"is_installed\":0,\"is_returning\":0,\"is_renting\":0,\"last_reported\":1519703208,\"station_id\":66505513,\"numbikesavailable\":0}"}
{"@version":"1","@timestamp":"2018-03-19T18:24:01.919Z","fields":"{\"numdocksavailable\":0,\"is_installed\":0,\"is_returning\":0,\"is_renting\":0,\"last_reported\":1519703208,\"station_id\":66505511,\"numbikesavailable\":0}"}
    ...

```

I want to add at root the field `fields.station_id`

The code bellow does not work:  
`filter { mutate { add_field => { "station_id" => "%{[fields][station_id]}"} } }`

Actually it create a field `station_id` with value literrally without interpretion of value  
`"station_id":"%{[fields][station_id]}"`

Please, could you help me to improve my selector?

---

<div class="post-metadata">

### Author: ![magnusbaeck](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/magnusbaeck/32/44943_2.png) [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [March 19, 2018, 7:02pm UTC](https://discuss.elastic.co/t/filter-mutate-add-field-select-a-value-in-an-inner-field/124626/2 "2018-03-19T19:02:29Z")

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Your `fields` field isn't an object with fields like `station_id`, it's a JSON string that needs to be parsed. Use a json filter for that.

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<div class="post-metadata">

### Author: ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)
#### Post date: [April 16, 2018, 7:02pm UTC](https://discuss.elastic.co/t/filter-mutate-add-field-select-a-value-in-an-inner-field/124626/3 "2018-04-16T19:02:53Z")

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