Unable to access field outside _source in Painless ingest processor

[marcosvrrs (Marcos V)]

Hi I can not figure out how to access a field that is outside the _source. The grok processor with the field nested I can't access, or the rename processor too. Tried to access the data with ctx, _source, _index. Don't know what to do.

POST /_ingest/pipeline/_simulate?verbose
{
  "pipeline" :
  {
    "description": "_description",
    "processors":[
  {
    "grok": {
      "field": "statusChangeHistory.L.M.changeRequestReceivedDate.S",
      "patterns": [
        "%{YEAR:ano}-%{MONTHNUM:mes}-%{MONTHDAY:dia}[T ]%{HOUR}:?%{MINUTE}(?::?%{SECOND})?%{ISO8601_TIMEZONE}"
      ],
      "ignore_missing": true,
      "ignore_failure": true
    }
  },
  {
    "rename": {
      "field": "_source.fields",
      "target_field": "field_xxx"
    }  
    
  }
  ]
  },
  "docs": [
    {
  "_index": "transaction_service",
  "_type": "_doc",
  "_id": "aut_e1c30967",
  
  "_score": null,
  "_source": {
    
    
    "statusChangeHistory": {
      
    }
  },
  "fields": {
    "statusChangeHistory.L.M.changeRequestReceivedDate.S": [
      "2021-01-05T13:48:34.496Z",
      "2021-01-05T13:49:06.000Z"
    ],
    "transactionDate.S": [
      "2021-01-05T03:00:00.000Z"
    ],
    "transactionReceivedDateTime.S": [
      "2021-01-05T13:48:34.496Z"
    ]
  },
  "sort": [
    1609854514496
  ]
}
  ]
}