title: mongodb index an array's key (not the values)
body: <p>In MongoDB, I have the following document</p>
  
  <pre><code>{
    "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
    "concepts": {
      "blabla": 20,
      "blibli": 100,
      "blublu": 250,
      ... (many more here)
    }
  }
  </code></pre>
  
  <p>And I would like to index it to be able to query for the "key" of the "concept" array (I know it's not really a mongoDB array...):</p>
  
  <pre><code>db.things.find({concepts:blabla});
  </code></pre>
  
  <p>Is it possible with the above schema? Or shall I refactor my documents to something like </p>
  
  <pre><code>{
    "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
    "concepts": ["blabla","blibli","blublu", ... (many more here)]
    }
  }
  </code></pre>