title: MongoDB Aggregation Function Returning Undefined
body: <p>I'm attempting to use use the new MongoDB aggregation features to tally some statistics by date.  Below is a sample of the documents that I am working with, my attempted code and desired result.  The aggregation function retuns "UNDEFINED".  Can someone tell me why that is? And secondly, I want my aggregation function to group results by date in mm-dd-yyyy format.  However as it is currently written I think the code is going to execute the aggregation by the full ISO date.  Can someone please tell me how to fix this?</p>
  
  
  
  <p>DOCUMENT EXAMPLE</p>
  
  <pre class="lang-javascript prettyprint-override"><code>{
    user: "2A8761E4-C13A-470E-A759-91432D61B6AF-25982-0000352D853511AF",
    language: "English",
    imageFileName: "F7A5ED9-D43C-4671-A5C6-F06C7E41F902-7758-000008371FB5B834",
    audioFileName: "F6D5727D-9377-4092-A28A-AA900F02653D-7758-0000083749066CF2",
    date: ISODate("2012-10-22T02:43:52Z"),
    correct: "1",
    _id: ObjectId("5084b2e8179c41cc15000001")
  }
  </code></pre>
  
  <p>AGGREGATION FUNCTION</p>
  
  <pre class="lang-javascript prettyprint-override"><code>    var getUserStats = function(user, language, callback) {
      var guessCollection = db.collection('Guesses');
      guessCollection.aggregate(
    { $match: {
      user: user,
      language: language,
    }},
    { $sort: {
      date: 1
    }},
    { $project : {
       user : 1,
              language : 1,
              date : 1,
              correct : 1,
              incorrect : 1,
    } },
    { $unwind : "$language" },
    { $group : {
       _id : "$date",
              correct : { $sum : "$correct" },
              incorrect : { $sum : "$incorrect" }
    } }
  , function(err, result){
      console.log(result);
      callback(result);
  });
  </code></pre>
  
  <p>DESIRED RESULT</p>
  
  <pre class="lang-javascript prettyprint-override"><code>  {
          "result" : [
      //...snip...
              {
                  "_id" : "2A8761E4-C13A-470E-A759-91432D61B6AF-25982-0000352D853511AF",
                  "correct" : 32,
                  "incorrect" : 17,
                  "date" : 2012-10-22
              },
  {
                  "_id" : "2A8761E4-C13A-470E-A759-91432D61B6AF-25982-0000352D853511AF",
                  "correct" : 16,
                  "incorrect" : 7,
                  "date" : 2012-10-23
              }
          ],
          "Ok" : 1
      }
  </code></pre>