title: Elasticsearch sorting by relationship
body: <p>I've defined relationship like this</p>
  
  <pre><code>PUT /my_index/user/1 
  {
    "name":     "John Smith",
    "email":    "john@smith.com",
    "dob":      "1970/10/24"
    "number_of_posts": 4
  }
  
  PUT /my_index/blogpost/2 
  {
    "title":    "Relationships",
    "body":     "It's complicated...",
    "user":     1 
  }
  </code></pre>
  
  <p>How can I sort blogpost by user information like name, email or number_of_posts without using nested field on blogpost, because I don't want to update all blogpost with user = 1 when user change their information. Thanks!</p>