Calculation across the root document and the nested objects

Elastic Stack Elasticsearch
1

I have a data model:

PUT /test
{
  "mappings": {
    "trade": {
      "properties": {
        "trade_from": {
          "type": "string",
          "index": "not_analyzed"
        },
        "trade_to": {
          "type": "string",
          "index": "not_analyzed"
        },
        "trade_gold": {
          "type": "integer"
        },
        "trade": {
          "type": "nested",
          "properties": {
            "item_name": {
              "type": "string",
              "index": "not_analyzed"
            },
            "item_count": {
              "type": "integer"
            }
          }
        }
      }
    }
  }
}

and data:

PUT /test/trade/1
{
  "trade_from": "PlayerA",
  "trade_to": "PlayerB",
  "trade_gold": 10000,
  "trade": [
    {
      "item_name": "PortionA",
      "item_count": 3
    },
    {
      "item_name": "PortionB",
      "item_count": 5
    },
    {
      "item_name": "PortionC",
      "item_count": 10
    },
  ]
}

and I want to get the average amount of gold per each item in the trading log, like:
PortionA | 3333.3333
PortionB | 2000
PortionC | 1000

I tried:

GET /test/_search
{
  "size": 0,
  "aggs": {
    "nested_2": {
      "nested": {
        "path": "trade"
      },
      "aggs": {
        "2": {
          "terms": {
            "field": "trade.item_name",
            "size": 10000,
            "order": {
              "_term": "desc"
            }
          },
          "aggs": {
            "1": {
              "avg": {
                "field": "trade.item_count",
                "script": "return doc['trade_gold'].value/doc['trade.item_count'].value;"
              }
            }
          }
        }
      }
    }
  }
}

I found the doc['trade_gold'] is null

and tried:

GET /test/_search
{
  "size": 0,
  "aggs": {
    "nested_2": {
      "nested": {
        "path": "trade"
      },
      "aggs": {
        "2": {
          "terms": {
            "field": "trade.item_name",
            "size": 10000,
            "order": {
              "_term": "desc"
            }
          },
          "aggs": {
            "nested_1": {
              "reverse_nested": {},
              "aggs": {
                "1": {
                  "avg": {
                    "field": "trade_gold",
                    "script": "return doc['trade_gold'].value/doc['trade.item_count'].value;"
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

I found doc['trade.item_count'] is null

How can I do the calculation across the root document and the nested objects ?

If there does not have solution, how the data model changed to achieve this calculation ?
As this is only the simplified version of my data model, I must keep the array of nested objects in one document, and cannot separate to three documents.