Trouble with nested object search in painless

Elastic Search
1

Hello, I am trying to do the following. Suppose there is an index called my-index with the following mapping:

{
  "my-index": {
    "mappings": {
      "properties": {
        "fields": {
          "type": "nested",
          "properties": {
            "id": {
              "type": "keyword"
            },
            "names": {
              "type": "keyword"
            }
          }
        }
      }
    }
  }
}

What I am trying to do is sort the documents based on fields[fieldId].names, where fieldId will be given in the params dynamically. This is what I've tried from Python shell.

import requests

headers = {
    'Content-Type': 'application/json',
}

json_data = {
    'script': {
        'source': """
            // Simplified version of an attempt
            if (doc['fields'] == null || doc['fields'].length == 0) {
              return false;
            }
            
            return true;

        """,
        'params': {
            'fieldId': 'id1'
        },
        'lang': 'painless',
    },
    'context': 'filter',
    'context_setup': {
        'index': 'my-index',
        'document': {
            'fields': [
                {
                    'id': 'id1',
                    'names': 'CF1,CF3',
                },
                {
                    'id': 'id2',
                    'names': [],
                },
                {
                    'id': 'id3',
                    'names': 'hello,world,lol,zzz',
                },
            ],
        },
    },
}

requests.post('http://localhost:9200/_scripts/painless/_execute?pretty', headers=headers, json=json_data).json()

I did not write the full query in the source. The main problem I am having is that whenever I try to access doc['fields'], it's throwing an error no matter what I try. And I can't figure out why it's happening and how to solve it. I can access doc['field.id'] and doc['field.names'] but not doc['field']. Any idea why I could be getting the following error and how to solve it? Any help would be greatly appreciated, thanks.

{'error': {'root_cause': [{'type': 'script_exception', 'reason': 'runtime error', 'script_stack': ['org.opensearch.search.lookup.LeafDocLookup.get(LeafDocLookup.java:87)', 'org.opensearch.search.lookup.LeafDocLookup.get(LeafDocLookup.java:55)', "if (doc['fields'] == null || doc['fields'].length == 0) {\n              ", '        ^---- HERE'], 'script': ' ...', 'lang': 'painless', 'position': {'offset': 69, 'start': 61, 'end': 133}}], 'type': 'script_exception', 'reason': 'runtime error', 'script_stack': ['org.opensearch.search.lookup.LeafDocLookup.get(LeafDocLookup.java:87)', 'org.opensearch.search.lookup.LeafDocLookup.get(LeafDocLookup.java:55)', "if (doc['fields'] == null || doc['fields'].length == 0) {\n              ", '        ^---- HERE'], 'script': ' ...', 'lang': 'painless', 'position': {'offset': 69, 'start': 61, 'end': 133}, 'caused_by': {'type': 'illegal_argument_exception', 'reason': 'No field found for [fields] in mapping'}}, 'status': 400}
2

Removed #opensearch