title: How to get all field names in elasticsearch index
body: <p>I just start using elasticsearch 5.2 .</p>
  
  <p>I am trying to get all keys in the index 
   if I have the following mapping:</p>
  
  <pre><code>"properties": {
           "name": { "type": "text" },
           "article": {
            "properties": {
             "id": { "type": "text" },
             "title":  { "type": "text"},
             "abstract": { "type": "text"},
              "author": {
               "properties": {
                "id": { "type": "text" },
                "name": { "type": "text" }
  }}}} } }
  </code></pre>
  
  <p><strong>is it possible to get all fields full name ?</strong>
   like this:</p>
  
  <pre><code> name,
   article.id ,
   article.title ,
   article.abstract ,
   article.author.id,
   article.author.name
  </code></pre>
  
  <p>how can I get that ?</p>