title: Check if an array key in a field exists in mongo
body: <pre><code>{
   "_id": ObjectId("502626aad4ebdf600b000000"),
   "created_at": ISODate("2012-08-11T09: 32: 26.0Z"),
   "excerpt": "lotto results for August 11, 2012",
   "results": {
      "1-digit" : {
          "0" : "1",
      },
      "2-digit" : {
          "0" : "1",
          "1" : "2",
      },
      "3-digit" : {
          "0" : "1",
          "1" : "2",
          "2" : "3",
      }
      }
  }
  </code></pre>
  
  <p>How should I check if <code>results.1-digit</code> exists?</p>