title: How to count number of objects in a nested field in elastic search?
body: <p>How to count number of objects in a nested filed in elastic search?
  Sample mapping :</p>
  
  <pre><code>"base_keywords": {
    "type": "nested",
    "properties": {
      "base_key": {
        "type": "text"
      },
      "category": {
        "type": "text"
      },
      "created_at": {
        "type": "date"
      },
      "date": {
        "type": "date"
      },
      "rank": {
        "type": "integer"
      }
    }
  }
  </code></pre>
  
  <p>I would like to count number of objects in nested filed 'base_keywords'.</p>