make couchDB calculate an average and keep it up to date when adding values
<p>Is it possible to make couchDB (Cloudant) calculate the average <code>rating</code>  value of each document ?
If yes, how to do this in Cloudant ?</p>

<p>Thanks.</p>

<pre><code>{
  "_id": "2016-02-06T13:16:30.515Z",
  "_rev": "3-7521b9e21fbb58d5393a76d08b12ab12",
  "modelStrict": "Taj model",
  "ratings": {
    "2016-02-06T13:36:04.671Z": {
      "userEmail": "lourain@yaho.fr",
      "rating": 1,
      "sessionNumber": 0
    },
    "2016-02-06T13:46:04.671Z": {
      "userEmail": "louin@yaoo.fr",
      "rating": 3,
      "sessionNumber": 0
    },
    "2016-02-06T13:53:04.671Z": {
      "userEmail": "luin@yaoo.fr",
      "rating": 3,
      "sessionNumber": 0
    }, 
    "2016-02-06T13:47:04.671Z": {
      "userEmail": "loin@yaoo.fr",
      "rating": 5,
      "sessionNumber": 0
    }
  },
  "averageRating": ...
}
</code></pre>

<p>And how to keep this averageRating up to to date when a rating is added ?</p>

<p>An added rating looks like this:</p>

<pre><code>    "2016-02-06T17:57:04.671Z": {
      "userEmail": "loin@yaoo.fr",
      "rating": 2,
      "sessionNumber": 10
    }
</code></pre>