Use reciprocal ranking fusion to combine the results of two queries

[flando]

Hi everyone,
I'm trying to use reciprocal ranking fusion (RRF) to combine the results of two query performed with the following code:

GET /books_index/_search
{
  "query": {
    "bool": {
      "should": [
        {
          "match": {
            "title": "the"
          }
        },
        {
          "match": {
            "description": "in"
          }
        }
      ]
    }
  }
}

In practice what I need to do is returning a single list of document based on the new score obtained by computing the RRF of each document.

The new score of each document should be: 1/(document position in the result list of the query on title) + 1/(document position in the result list of the query on description).

Maybe it can be done with scripted_metric? But I do not know how.
I was also thinking on using aggregation, but I did not succeeded.

[flando]

I have found that there is an open pull request.

There is no way to get it with function_score/script_score?