title: Logstash Dynamic Index From Document Field Fails
body: <p>I still face problems to figure out, how to tell Logstash to send a dynamic index, based on a document field. Furthermore, this Field must be transformed in order to get the "real" index at the very end.
  Given, that there is a field "time" (which is a UNIX Timestamp). This Field gets already transformed with a "date" Filter to a DateTime Object for Elastic.
  Additionally, it should server as index (YYYYMM). The index should NOT be derived from @Timestamp, which is not touched.</p>
  
  <p>Example:
  {...,"time":1453412341,...}</p>
  
  <p>Shall go to the Index: 201601</p>
  
  <p>I use the following Config:</p>
  
  <pre><code>filter {
    date {
      match =&gt; [ "time", "UNIX" ]
      target =&gt; "time"
      timezone =&gt; "Europe/Berlin"
    }
  }
  output {
      elasticsearch {
          index =&gt; "%{time}%{+YYYYMM}"
          document_type =&gt; "..."
          document_id =&gt; "%{ID}"
          hosts =&gt; "..."
      }
  }
  </code></pre>
  
  <p>Sadly, its not working. Any idea, how to achieve that?</p>
  
  <p>Thanks a lot!</p>