title: How to return array of string with mongodb aggregation
body: <p>I need to return array of string with mongodb aggregation. I did the following:</p>
  
  <pre><code>db.users.aggregate([{$group: {_id:"$emails.address"}}])
  </code></pre>
  
  <p>It return:</p>
  
  <pre><code>{ "_id" : [ "a@a.com" ] }
  { "_id" : [ "b@a.com" ] }
  { "_id" : [ "c@a.com" ] }
  </code></pre>
  
  <p>Is there a way to return array of string like this one:</p>
  
  <pre><code>["a@a.com","b@a.com","c@a.com"]
  </code></pre>
  
  <p>thank You very much anyone who taking your time for helping me..</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Adding data:</p>
  
  <pre><code>{
      "_id" : "ukn9MLo3hRYEpCCty",
      "createdAt" : ISODate("2015-10-24T03:52:11.960Z"),
      "emails" : [
          {
              "address" : "a@a.com",
              "verified" : false
          }
      ]
  }
  {
      "_id" : "5SXRXraariyhRQACe",
      "createdAt" : ISODate("2015-10-24T03:52:12.093Z"),
      "emails" : [
          {
              "address" : "b@a.com",
              "verified" : false
          }
      ]
  }
  {
      "_id" : "WMHWxeymY4ATWLXjz",
      "createdAt" : ISODate("2015-10-24T03:52:12.237Z"),
      "emails" : [
          {
              "address" : "c@a.com",
              "verified" : false
          }
      ]
  }
  </code></pre>