Sum of min_by in Elasticsearch

aggregation.json

GET testindex/_search
{
  "size": 0,
  "aggs": {
    "names": {
      "terms": {
        "field": "name.keyword"
      },
      "aggs": {
        "oldest_amount": {
          "scripted_metric": {
            "map_script": """
              if (state.oldest_date == null 
                  || doc.date.value.isBefore(state.oldest_date)) {
                state.oldest_date = doc.date.value;
                state.amount = doc.amount.value;
              }""",
            "reduce_script": """
              return states
                .stream()
                .min(Comparator.comparing(i->i.oldest_date))
                .map(i->i.amount)
                .get()"""
          }
        }
      }
    },
    "total_amount": {
      "sum_bucket": {
        "buckets_path": "names>oldest_amount.value"
      }
    }
  }
}