Inconsistent handling of empty strings in numerical fields between ingestion and fetch

[iverase]

If we try to index a document with an empty string on a field that is declared numerical, e.g. an integer, the document is ingested without issues:

PUT test
{
  "mappings": {
    "properties": {
      "numerical_field": {
        "type": "integer"
      }
    }
  }
}
PUT test/_doc/1
{
  "numerical_field": ""
}

This is due to coerce being true by default which makes empty strings to be treated as null values. This behaviour is not documented and should be added to the docs.

The issue is when we try to fetch this document using the fields API:

GET test/_search
{
  "_source": false, 
  "fields": ["numerical_field"]
}

{
  "took" : 0,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "test",
        "_id" : "1",
        "_score" : 1.0,
        "ignored_field_values" : {
          "numerical_field" : [
            ""
          ]
        }
      }
    ]
  }
}

The field is treated as malformed and added to ignored fields which seems inconsistent whit the ingestion time behaviour.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)