stats aggregation on list length

[Jilles van Gurp]

I'm trying to do a stats aggregation on the list length using a script but 
I'm getting errors. For this data, 

PUT test_groups/group/1
{
  "name":"1",
  "members":[
    {
      "name":"m1"
    }
  ]
}

PUT test_groups/group/2
{
  "name":"2",
  "members":[
    {
      "name":"m1"
    },
    {
      "name":"m2"
    }
  ]
}

and this query:

GET test_groups/group/_search
{
  "aggs": {
    "group_members": {
      "filter": {
        "exists": {
          "field": "members"
        }
      },
      "aggs": {
        "length": {
          "stats": {
            "script": "doc['members'].values.length"
          }
        }
      }
    }
  }
}

I get an error stating that the members field does not exist in type group:

{
   "took": 4,
   "timed_out": false,
   "_shards": {
      "total": 5,
      "successful": 3,
      "failed": 2,
      "failures": [
         {
            "index": "test_groups",
            "shard": 2,
            "status": 500,
            "reason": "QueryPhaseExecutionException[[test_groups][2]: 
query[ConstantScore(cache(_type:group))],from[0],size[10]: Query Failed 
[Failed to execute main query]]; nested: 
GroovyScriptExecutionException[ElasticsearchIllegalArgumentException[No 
field found for [members] in mapping with types [group]]]; "
         },
         {
            "index": "test_groups",
            "shard": 3,
            "status": 500,
            "reason": "QueryPhaseExecutionException[[test_groups][3]: 
query[ConstantScore(cache(_type:group))],from[0],size[10]: Query Failed 
[Failed to execute main query]]; nested: 
GroovyScriptExecutionException[ElasticsearchIllegalArgumentException[No 
field found for [members] in mapping with types [group]]]; "
         }
      ]
   },
   "hits": {
      "total": 0,
      "max_score": null,
      "hits": []
   },
   "aggregations": {
      "group_members": {
         "doc_count": 0,
         "length": {
            "count": 0,
            "min": null,
            "max": null,
            "avg": null,
            "sum": null
         }
      }
   }
}

Is there a way to do this?

Best regards,

Jilles

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