title: MongoDB alternative to $exists
body: <p>I have 100000 documents and I need to retrieve the top 10 documents that have the highest values.</p>
  
  <p>Here's an example of a simplified document:</p>
  
  <pre><code>{
      "_id" : ObjectId( "4f2fbeb0369a2ba603000000" ),
      "usage" : {
          "2012" : {
              "2" : {
                  "project" : {
                      "4f182a76369a2b2903000000" : {
                          "site" : {
                              "1" : 5,
                              "2" : 1
                          }
                      }
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>The value that the sort/limit will be applied to can be found in 'usage.2012.2.project.4f182a76369a2b2903000000.site.1' (in the case of the above example, the value is 5).</p>
  
  <p>The problem is that in some documents this object exists, in others it won't.</p>
  
  <p>Now I could use $exists to check if the object exists, but according to the MongoDB manual using $exists is inefficient. Are there any alternative ways that I could go about doing this? I can change the schema if required to suit..</p>