ElasticSearch custom script score that does not replace the natural scoring

Question

I have an ElasticSearch query with a custom script score written in Painless language. For now the ES request looks like this, with the natural ES _score being totally replaced by a custom scoring from my script:

{
  "_source": {
    "excludes": [
      "field_to_exclude",
    ]
  },
  "from": 0,
  "size": 100,
  "query": {
    "script_score": {
      "query": {
        "bool": {
          "must": {
            "match_all": {}
          },
          "filter": [
            {
              "term": {
                "field_to_filter": 4
              }
            }
          ]
        }
      },
      "script": {
        "lang": "painless",
        "source": "COMPLEX_PAINLESS_SCRIPT"
      }
    }
  },
  "sort": [
    {
      "price": {
        "order": "asc"
      }
    },
    "_score"
  ]
}

Depending on some parameter from the frontend, I want to be able to still calculate the ES natural scoring separately, and keep this custom scoring to be calculated in another field, and even if possible being used as a secondary sorting criteria.

Is this possible?

Answer 1

I finally figured out. This can actually be done if you use a custom _script field in the sort array, along with the _score field for the natural ES score.

And aside from that, we can keep the custom scoring in a specific field in the response by using script_fields.

Both the ES requests, depending on the sorting criteria, would look like:

{
  "_source": {
    "excludes": [
      "field_to_exclude",
    ]
  },
  "from": 0,
  "size": 100,
  "query": {
    "bool": {
      "must": {
        "match_all": {}
      },
      "filter": [
        {
          "term": {
            "field_to_filter": 4
          }
        }
      ]
    }
  },
  "sort": [
    {
      "price": {
        "order": "desc"
      }
    },
    {
      "_score": {
        "order": "desc"
      }
    },
    {
      "_script": {
        "type": "number",
        "script": {
          "lang": "painless",
          "source": "COMPLEX_PAINLESS_SCRIPT"
        },
        "order": "desc"
      }
    }
  ],
  "script_fields": {
    "custom_score": {
      "script": {
        "lang": "painless",
        "source": "COMPLEX_PAINLESS_SCRIPT"
      }
    }
  }
}

and

{
  "_source": {
    "excludes": [
      "field_to_exclude",
    ]
  },
  "from": 0,
  "size": 100,
  "query": {
    "bool": {
      "must": {
        "match_all": {}
      },
      "filter": [
        {
          "term": {
            "field_to_filter": 4
          }
        }
      ]
    }
  },
  "sort": [
    {
      "_script": {
        "type": "number",
        "script": {
          "lang": "painless",
          "source": "COMPLEX_PAINLESS_SCRIPT"
        },
        "order": "desc"
      }
    },
    {
      "_score": {
        "order": "desc"
      }
    }
  ],

  "script_fields": {
    "fit_score": {
      "script": {
        "lang": "painless",
        "source": "COMPLEX_PAINLESS_SCRIPT"
      }
    }
  }
}