Different behavior fields in nested object. If store="yes", Elasticsearch does not return stored data. If store="no", Elasticsearch return stored data from _source.

result_of_query.json

{
   "took": 3,
   "timed_out": false,
   "_shards": {
      "total": 5,
      "successful": 5,
      "failed": 0
   },
   "hits": {
      "total": 1,
      "max_score": 0.625,
      "hits": [
         {
            "_index": "library",
            "_type": "books",
            "_id": "1",
            "_score": 0.625,
            "_source": {
               "book": {
                  "title": "ElasticSearch Server",
                  "contents": "Learn the basics of ElasticSearch"
               }
            },
            "fields": {
               "book.title": [
                  "ElasticSearch Server"
               ]
            }
         }
      ]
   }
}

return_fields_in_nested_object.json

PUT /library

POST /library/books/_mapping
{
  "books" : {
    "properties": {
      "book" : {
        "type": "nested",
        "properties": {
          "title" : { "type": "string", "analyzer": "standard", "store": "no"},
          "contents" : {"type": "string", "analyzer": "standard", "store": "yes"}
        }
      }
    }
  }
}

PUT /library/books/1
{
  "book" : {
    "title" : "ElasticSearch Server", 
    "contents" : "Learn the basics of ElasticSearch"
  }
}

GET /library/books/_search
{
  "_source" : ["book.title","book.contents"],
  "fields": [
    "book.title",
    "book.contents"
  ], 
  "query": {
    "nested": {
      "path": "book",
      "query": {
        "query_string" : {
          "query" : "ElasticSearch",
          "fields" : ["book.title", "book.contents"]
        }
      }
    }
  }
}