Using Nested Field in Composite Aggregation

[michael-jaxsta (Michael Stone)]

This question has been asked a few times in this community but have not got a response so hopefully someone reads this and helps me out!

I'm trying to do a composite aggregation on documents which have nested fields. Specifically I want one of the nested fields to be part of the composite bucket.

My documents are related to musical charts where each document is a specific song on a specific chart for a specific date and look similar to the example below

{ 
   "provider": "Billboard",
   "start_date": "2023-03-23",
   "title": "Shake it Off",
   "artist": "Taylor Swift",
   "position": 1,
   "credits": 
   [
      { "name": "Jack Antonoff", "role": "Producer"},
      { "name": "Max Martin", "role": "Songwriter"},
   ]
}       

I want to be able to create a composite bucket aggregation including provider, title, artist, credits.name and credits.role. There is then a metric subaggregation for MIN(position) i.e. the best chart position each of the credited people achieved for a given song on a given chart.

The query below works but as soon as I add the nested field I get no hits.

{
  "aggs": {
    "my_buckets": {
      "composite": {
        "size": 1000,
        "sources": [
          { "title": { "terms": { "field": "title"}}},
          { "artist": { "terms": { "field": "artist"}}},
          { "provider": { "terms": { "field": "provider"}}},
        ]
      },
      "aggregations": {
        "peak_position": { "min": { "field": "position" }
        }
      }
    }
  }
}

Thanks!

Michael