ElasticSearch aggregate on a scripted nested field

Elastic Stack Elasticsearch
2

I've managed to figure this out in the end.

The two things I hadn't realised were:

  1. You can provide a script instead of a field key to bucket aggregations.
  2. Instead of using nested queries, you can access nested values directly using params._source.

The combination of these two things allowed me to write the correct query:

{
  "size": 0,
  "aggs": {
    "max.float_property": {
      "terms": {
        "script": "double max = 0; for (item in params._source.entities) { if (item.float_property > max) { max = item.float_property; }} return max;"
      }
    }
  }
}

Response:

{
  ...
  "aggregations": {
    "max.float_property": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "0.9",
          "doc_count": 2
        },
        {
          "key": "0.6",
          "doc_count": 1
        }
      ]
    }
  }
}

I'm confused though, because I thought the correct way to access nested fields was by using the nested query type. Unfortunately there's very little documentation for this, so I'm still unsure if this is the intended/correct way to aggregate on scripted nested fields.