random-score.json

GET /raffle/_search
{
    "size": 1,
    "query": {
        "function_score": {
            "functions": [
                {
                    "random_score":  {  }
                }
            ],
            "score_mode": "sum"
        }
    }
}

In 6.4.0, make sure to run a forcemerge to have only one segment when you have few documents.
Example:

DELETE raffle
PUT raffle
{
  "settings": {
    "number_of_shards": 1
  }
}
POST raffle/_doc
{
  "name": "User 1"
}
POST raffle/_doc
{
  "name": "User 2"
}
POST raffle/_doc
{
  "name": "User 3"
}
POST raffle/_forcemerge?max_num_segments=1
GET /raffle/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "random_score": { }
        }
      ],
      "score_mode": "sum"
    }
  }
}