Composite aggregation ignores value script in terms source

[hendrikmuhs]

Using a terms source in a composite aggregation is documented to fully support scripts in the same way as terms aggregation:

The terms value source is equivalent to a simple terms aggregation. The values are extracted from a field or a script exactly like the terms aggregation.

However, value script does not work as expected but is silently ignored.

Workaround

Instead of using _value use doc[{field_name}]

Reproduction

The described behavior can be reproduced using the kibana sample data logs:

GET kibana_sample_data_logs/_search
{
  "size": 0,
  "aggs": {
    "my_composite_agg": {
      "composite": {
        "sources": [
          {
            "agent": {
              "terms": {
                "field": "agent.keyword",
                "script": {
                  "source": "'agent: ' + _value"}
              }
            }
          }
        ]
      }
    }
  }
}

returns e.g.

      "buckets" : [
        {
          "key" : {
            "agent" : "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)"
          },
          "doc_count" : 4010
        },

while the expected output is:

      "buckets" : [
        {
          "key" : {
            "agent" : "agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)"
          },
          "doc_count" : 4010
        },

I am not sure if this is really a bug or a technical limitation. If its a limitation the documentation should not claim full support and the search request using a value script should error.

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[hendrikmuhs]

I debugged this to learn a bit about the implementation of aggs. #53232 contains what I found out and also has a quick-fix for it.

[bczifra]

@hendrikmuhs I've run into field+script issues before, such as #43390. Pulling the entire content into the script will work reliably:

GET kibana_sample_data_logs/_search
{
  "size": 0,
  "aggs": {
    "my_composite_agg": {
      "composite": {
        "sources": [
          {
            "agent": {
              "terms": {
                "script": {
                  "source": "'agent: ' + doc['agent.keyword']"}
              }
            }
          }
        ]
      }
    }
  }
}

[hendrikmuhs]

@bczifra Thanks. yes using doc works, I should have mentioned this for readers of the issue. I will update the entry post.

[wchaparro]

closing as not planned.