title: Aggregate sum of values in Mongodb
body: <p><strong>Here is My Collection:</strong></p>
  <pre><code>{
  rate:4
  },
  {
  rate:6
  },
  {
  rate:7
  }
  </code></pre>
  <p>I'm using Mongodb and I want calculate the sum of rate, i.e <strong>4+6+7=17</strong>.</p>