title: Adding new element to array in MongoDb
body: <p>I have a mongodb collection like this:-</p>
  
  <pre><code>{
      "_id": ObjectId("52174bcb834806830e5447"),
      "roles": [
          {
              "role": "admin"
          },
          {
              "role": "user"
          }
      ]
  }
  </code></pre>
  
  <p>I need to add a new 'role' to the <code>roles</code> array. Like this <code>{"role": "guest" }</code>. How do I do that?</p>