title: MongoDB aggregation projection on collection
body: <p>I have a collection full of documents like this. </p>
  
  <pre><code>{
      "_id" : ObjectId("5696db8a049c1bb2ecaaa10f"),
      "resultSets" : [
          {
              "headers" : [ "A", "B" ],
              "data" : [
                  [ 0, 1 ]
              ]
          },
  
          {
              "headers" : [ "C", "D" ],
              "data" : [
                  [ 2, 3 ]
              ]
  
          }
     ]
  }
  </code></pre>
  
  <p>I would like to use aggregation to project this document as just. </p>
  
  <pre><code>{ C: 2, D: 3 }
  </code></pre>
  
  <p>Just wondering what the best way to do this would be.</p>