Reference of update by query adding tag into doc

elasticsearch_update_by_query_6.2.es

GET /<something>/_update_by_query?pretty
{
  "query": {
    "bool": {
      "must_not": [
        {
          "match_phrase": {
            "tags.keyword": {
              "query": "parsed"
            }
          }
        },
        {
          "match_phrase": {
            "tags.keyword": {
              "query": "unparsed"
            }
          }
        }
      ]
    }
  },
  "script": {
    "source": "if (ctx._source.tags == null) { ctx._source.tags = [params.tag]}else{ctx._source.tags.add(params.tag)}",
    "params": {
      "tag": "unparsed"
    }
  }
}