Define array of strings field in elastic search

Question

How to define a field that accepts an array of strings like ["A", "B", "C"].

I have tried to do the following: crated a field in my index:

    {
      "properties": 
      {
        "date": {"type": "date"},
        "imageUrls": { "type": "nested" },
        }
    }

And I write the document

..../_doc/1

method: POST

body:

{
    "imageUrls": ["A", "B", "C"]
}

Always getting this error:

{
    "error": {
        "root_cause": [
            {
                "type": "mapper_parsing_exception",
                "reason": "object mapping for [imageUrls] tried to parse field [null] as object, but found a concrete value"
            }
        ],
        "type": "mapper_parsing_exception",
        "reason": "object mapping for [imageUrls] tried to parse field [null] as object, but found a concrete value"
    },
    "status": 400
}

Answer 1

Single values in a nested mapping are not allowed. From the docs -- the system

allows arrays of objects to be indexed in a way that they can be queried independently of each other.

That is to say,

PUT ahm
{
  "mappings": {
    "properties": {
      "date": {
        "type": "date"
      },
      "imageUrls": {
        "type": "nested",
        "properties": {
          "url": {
            "type": "text",
            "fields": {
              "keyword": {
                "type": "keyword"
              }
            }
          }
        }
      }
    }
  }
}

and then

POST ahm/_doc
{
  "imageUrls": [
    {
      "url": "A"
    },
    {
      "url": "B"
    },
    {
      "url": "C"
    }
  ]
}

This is quite nonsensical as-is but if more properties were added to the array objects, it would start to make sense.