Nested type in Elasticsearch: "object mapping can't be changed from nested to non-nested" when indexing a document
<p>I try to index some nested documents into an Elasticsearch (v2.3.1) mapping which looks as follows <a href="https://www.elastic.co/guide/en/elasticsearch/guide/current/nested-mapping.html" rel="noreferrer">(based on this example from the documentation)</a>:</p>

<pre><code>PUT /my_index
{
  "mappings": {
    "blogpost": {
      "properties": {
        "title": { "type": "string" },
        "comments": {
          "type": "nested", 
          "properties": {
            "name":    { "type": "string"  },
            "comment": { "type": "string"  }
          }
        }
      }
    }
  }
}
</code></pre>

<p>However, I do not understand what my JSON documents have to look like in order to fit into that mapping. I tried with</p>

<pre><code>PUT /my_index/some_type/1
{
  "title": "some_title",
  "comments": {
    "name": "some_name",
    "comment": "some_comment"
  }
}
</code></pre>

<p>as well as with</p>

<pre><code>PUT /my_index_some_type/1
{
  "title": "some_title",
  "comments": [
      {
        "name": "some_name",
        "comment": "some_comment"
      }
  ]
}
</code></pre>

<p>which both result in </p>

<pre><code>{

    "error": 

{

    "root_cause": 

[

            {
                "type": "remote_transport_exception",
                "reason": "[Caiman][172.18.0.4:9300][indices:data/write/index[p]]"
            }
        ],
        "type": "illegal_argument_exception",
        "reason": "object mapping [comments] can't be changed from nested to non-nested"
    },
    "status": ​400

}
</code></pre>

<p>Which is the correct format to index nested documents? Any working examples are much appreciated, most examples here at SO or on other pages concentrate on nested queries rather than how the documents have been indexed before.</p>