title: MongoDB $group with nested object key $sum
body: <p>The object structure is as follows:</p>
  
  <pre><code>{
      "_id" : ObjectId("53434bfe234416601c8b4567"),
      "package" : {
          "id" : "3",
          "title" : "P3",
          "price" : "8.00",
          "votes" : "8",
          "created_at" : "2014-04-08 00:29:35",
          "updated_at" : "2014-04-08 00:31:39"
      },
      "status" : "success",
      "user_id" : "5341dd9c234416de758b4570",
      "updated_at" : ISODate("2014-04-08T01:08:52.168Z"),
      "created_at" : ISODate("2014-04-08T01:08:14.849Z")
  }
  </code></pre>
  
  <p>What I'm trying to do is collect the sum of <code>package.price</code> with this:</p>
  
  <pre><code>db.transactions.aggregate({
      $group:{
          _id: null,
          'total': {$sum: "$package.price"}
       }
  })
  </code></pre>
  
  <p>And the <code>total</code> is always <code>int 0</code>, here's the result:</p>
  
  <pre><code>{
      "result" : [ 
          {
              "_id" : null,
              "total" : 0
          }
      ],
  
      "ok" : 1
  }
  </code></pre>