title: Elasticsearch 2.0: _id is not configurable
body: <p>ES 2.0 is just released. Out of curiosity, I tested my mapping file that has been working for ES 1.7.3 against ES 2.0. I got the following error:</p>
  
  <pre><code>{
    "error": {
      "root_cause": [
        {
          "type": "mapper_parsing_exception",
          "reason": "_id is not configurable"
        }
      ],
      "type": "mapper_parsing_exception",
      "reason": "mapping [mydoctype]",
      "caused_by": {
        "type": "mapper_parsing_exception",
        "reason": "_id is not configurable"
      }
    },
    "status": 400
  }
  </code></pre>
  
  <p>Basically, this error complains about my mapping file which contains the following:</p>
  
  <pre><code>"mydoctype": {
      "_id" : {
          "path" : "id"
      },
  </code></pre>
  
  <p>Just did google and failed to find a way to set doc _id to the actual identifier of indexed document in ES 2.0.  How can I do that? </p>
  
  <p>Thanks for any pointer and input!</p>