title: Elasticsearch order by field in child document
body: <p>I have documents which look like this:<br></p>
  
  <pre><code>{
      "parent" : {
          "properties" : {
              "id": {
                  "type" : "keyword"
              }
              "name" : {
                  "type" : "keyword"
              }
          }
      }
  }
  </code></pre>
  
  <p>and child documents:</p>
  
  <pre><code>{
      "child": {
          "_parent": {
              "type": "parent"
          },
          "_routing": {
              "required": true
          },
          "properties": {
              "child_id": {
                  "type" : "keyword"
              },
              "child_name" : {
                   "type" : "keyword"
              },
              "child_time": {
                  "type": "date"
              }
          }
      }
  }
  </code></pre>
  
  <p>How can I sort parent documents by child_name of child with the latest child_time?</p>