Custom Similarity for Percolate Query

[aneeshkoya (Aneesh)]

I created a percolation index with a custom similarity in its settings and a mapping to use this custom similarity:

{
  "my-index": {
    "settings": {
      "index": {
        "similarity": {
          "idfless": {
            "type": "scripted",
            "script": {
              "source": "double tf = Math.sqrt(doc.freq); double idf = 1.0; double norm = 1/Math.sqrt(doc.length); return query.boost * tf * idf * norm;"
            }
          }
        }
      }
    },
    "mappings": {
      "my-doc-type": {
        "message": {
          "type": "text",
          "similarity": "idfless"
        },
        "query": {
          "type": "percolator"
        }
      }
    }

However, this similarity setting is not respected. If I run a query and ask it to be explained like:

POST my-index/search/0/_explain
{
  "query": {
    percolate" : {
      "field" : "query",
      "documents" : [
        {
          "message": ...
        },
        {
          "message": ...
        }
      ]
    }
  }
}

However, result obtained is like this:

{
  "value": 0.6931472,
  "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
  "details": [
    {
      "value": 1,
      "description": "docFreq",
      "details": []
    },
    {
      "value": 2,
      "description": "docCount",
      "details": []
    }
  ]
}

Is there something wrong that I'm doing in order to get my percolator to use the specified similarity? Similar question has been posted in https://discuss.elastic.co/t/custom-similarity-not-respected-in-percolate-query/239541, however it is not answered yet.