Sorting by _meta fields

[vsevolodk (Vsevolod Kaimashnikov)]

Hello, please help.
I have three indices A, B, C and I need multiple search with sorting by index order
B, A, C. So lexicographical sorting result is not suitable and I want create _meta field for order
B - 1
A - 2
C - 3

Adding _meta field

POST B
{
  "mappings": {
    "user": {
      "_meta": { 
        "sort_order" : 1
      }
    }
  }
}

Mapping

"mappings" : {
      "user" : {
        "_meta" : {
          "sort_order" : 1
        },
        "dynamic_templates" : [
          {
            "string_as_keyword" : {
              "match_mapping_type" : "string",
              "mapping" : {
                "fields" : {
                  "raw" : {
                    "type" : "keyword"
                  }
                },
                "type" : "keyword"
              }
            }
          }
        ],
        "properties" : {
          "foo" : {
            "type" : "keyword",
            "fields" : {
              "raw" : {
                "type" : "keyword"
              }
            }
          }
        }
      }

And do sorting via _meta.sorting_order.

POST /B/_search?
{
 "size" : 10000,
 "sort" : [
  { "_meta.sort_order" : {"order" : "asc"} }
 ]
}

But I get

"root_cause": [
            {
                "type": "query_shard_exception",
                "reason": "No mapping found for [_meta.sort_order] in order to sort on",
                "index_uuid": "hJS-64PbS76lZpJduVWhdw",
                "index": "B"
            }
        ]

What I do wrong?