title: ElasticSearch nested _count query gets "request does not support [filter]"
body: <p>I have a query that works fine when I run it using <code>_search</code>, but it fails when using <code>_count</code>.  Can someone tell me why?  I'd rather not have to run the complete query just to get a count. </p>
  
  <p>This is the query. </p>
  
  <pre><code>{
    "filter": {
      "nested": {
        "filter": {
          "bool": {
            "must": [
              {
                "term": {
                  "user_id": 5
                }
              }
            ]
          }
        }, 
        "path": "participants"
      }
    }
  }
  </code></pre>
  
  <p>This is the failure: </p>
  
  <pre><code>{
      "count": 0,
      "_shards": {
          "total": 5,
          "successful": 0,
          "failed": 5,
          "failures": [
              {
                  "index": "messages_20150428_000025",
                  "shard": 0,
                  "reason": "BroadcastShardOperationFailedException[[messages_20150428_000025][0] ]; nested: QueryParsingException[[messages_20150428_000025] request does not support [filter]]; "
              },
              {
                  "index": "messages_20150428_000025",
                  "shard": 1,
                  "reason": "BroadcastShardOperationFailedException[[messages_20150428_000025][1] ]; nested: QueryParsingException[[messages_20150428_000025] request does not support [filter]]; "
              },
              {
                  "index": "messages_20150428_000025",
                  "shard": 2,
                  "reason": "BroadcastShardOperationFailedException[[messages_20150428_000025][2] ]; nested: QueryParsingException[[messages_20150428_000025] request does not support [filter]]; "
              },
              {
                  "index": "messages_20150428_000025",
                  "shard": 3,
                  "reason": "BroadcastShardOperationFailedException[[messages_20150428_000025][3] ]; nested: QueryParsingException[[messages_20150428_000025] request does not support [filter]]; "
              },
              {
                  "index": "messages_20150428_000025",
                  "shard": 4,
                  "reason": "BroadcastShardOperationFailedException[[messages_20150428_000025][4] ]; nested: QueryParsingException[[messages_20150428_000025] request does not support [filter]]; "
              }
          ]
      }
  }
  </code></pre>
  
  <p>Given this the mapping for this nested field:</p>
  
  <pre><code>"participants": {
      "type": "nested",
      "properties": {
          "archived": {
              "type": "boolean"
          },
          "has_unread": {
              "type": "boolean"
          },
          "name": {
              "type": "string"
          },
          "pk": {
              "type": "long"
          },
          "user_id": {
              "type": "long"
          }
      }
  },
  </code></pre>
  
  <p>and these data (for this nested field only):</p>
  
  <pre><code>"participants": [
      {
          "archived": false,
          "user_id": 5,
          "name": "Person A",
          "has_unread": false,
          "pk": 1
      },
      {
          "archived": false,
          "user_id": 7,
          "name": "Person B",
          "has_unread": false,
          "pk": 2
      }
  ],
  </code></pre>