Search as you type missing field - problem

[l-trotta]

Reported in the java repo.
Search as you type doesn’t support doc_values as a field, in fact trying to add it will result in a server error:

PUT my-index-sayt
{
  "mappings": {
    "properties": {
      "my_field": {
        "type": "search_as_you_type",
        "doc_values": true
      }
    }
  }
}

returns

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Cannot set [doc_values] on field of type [search_as_you_type]"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Failed to parse mapping: Cannot set [doc_values] on field of type [search_as_you_type]",
    "caused_by": {
      "type": "mapper_parsing_exception",
      "reason": "Cannot set [doc_values] on field of type [search_as_you_type]"
    }
  },
  "status": 400
}

But then, after a field mapper "search_as_you_type" has been correctly created, a GET will always have doc_values set as false:

{
  "my-index-sayt": {
    "mappings": {
      "properties": {
        "my_field": {
          "type": "search_as_you_type",
          "doc_values": false,
          "max_shingle_size": 3
        }
      }
    }
  }
}

There's a justification for this in a comment in the server code.

Possible ways to handle this:

1. Just add the field, response will deserialize but requests could be sent wrong
2. Use AdditionalProperty

I don't think we can have a separate response class since SearchAsYouTypeProperty is part of the Property variant and would complicate things a lot.

Field definition:

  /**
   * Cannot be set, will only be returned as false
   * @server_default false
   */
  doc_values?: boolean