title: Find documents where value is not null
body: <p>I have the following doc structure</p>
  
  <pre><code>{
      "model" : {
  
      },
      "_id" : ObjectId("538df963d1c3f82329000257"),
      "email" : "sam@yahoo.com",
      "name" : "sam",
      "endpointarn" : "arn:aws:sns:us-east-1:284585229836:endpoint/GCM/Positivethinking/59eb7c66-c13c-3e44-af80-0b1f4f7ea9fd",
      "devregidtoken" : "APA91bFMOBo6ZWemMAG5clKEd5VYVvbeDOM5zNXjbbl3faSE0FZt3gtJYv0hpjdWuOY-mvr-k3sCcq0dEveCM6jx5mOh1u6JEubuUmKb2zU64dn_A4gJ4pCBG7AGQJ8DnkO83Ca4wgzsoJcio9T-TtA",
      "topicsubs" : [
          {
              "topicsubid" : "arn:aws:sns:us-east-1:284585229836:Wealth-ProsperityQuotes:5f3e8060-48fa-4a8e-bdc3-e9596747da1a",
              "topicname" : "Wealth-Prosperity",
              "topicno" : 0
          }
      ],
      "timecreated" : ISODate("2014-06-03T16:35:47.442Z"),
      "purchasedata" : {
          "orderid" : "111",
          "packagename" : "",
          "productid" : "",
          "purchasetime" : "",
          "purchasestate" : "",
          "developerpayload" : "",
          "purchasetoken" : ""
      },
      "ccode" : "",
      "ccodestat" : ""
  }
  </code></pre>
  
  <p>I want to get all documents where purchasedata.orderid IS NOT NULL. I have tried </p>
  
  <pre><code>db.User.find({"purchasedata.orderid" : {$ne : ""}});
  </code></pre>