http://stackoverflow.com/questions/42862288/elasticsearch-aggregates-on-nested-fields

results.json

{
   "took": 55,
   "timed_out": false,
   "_shards": {
      "total": 5,
      "successful": 5,
      "failed": 0
   },
   "hits": {
      "total": 2,
      "max_score": 2.3527396,
      "hits": [
         {
            "_index": "newindex3",
            "_type": "article",
            "_id": "1",
            "_score": 1.4862027,
            "_source": {
               "title": "Some Title",
               "content": "FIY Fix yourself",
               "tags": [
                  {
                     "tagName": "Fix it yourself"
                  },
                  {
                     "tagName": "customer tips"
                  },
                  {
                     "tagName": "competition"
                  }
               ]
            }
         }
      ]
   },
   "aggregations": {
      "tags": {
         "doc_count": 6,
         "tags-tagnames": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
               {
                  "key": "competition",
                  "doc_count": 2
               },
               {
                  "key": "customer tips",
                  "doc_count": 2
               },
               {
                  "key": "Fix it yourself",
                  "doc_count": 1
               },
               {
                  "key": "how to",
                  "doc_count": 1
               }
            ]
         }
      }
   }
}