title: Querying mongodb with $in not working
body: <p>Why is this query not working?</p>
  
  <pre><code>&gt;db.fs.files.find()
  { "_id" : ObjectId("550a34129c44a4fa03fb78ea"), "filename" : "mJnB2DhK.png", "contentType" : "binary/octet-stream", "length" : 28297, "chunkSize" : 261120, "uploadDate" : ISODate("2015-03-19T02:27:30.757Z"), "aliases" : null, "metadata" : { "id" : "mJnB2DhK" }, "md5" : "85944f92db940c6590fc8059db91ea16" }
  { "_id" : ObjectId("550a34ed48f6205504285a3b"), "filename" : "mkPQ6wnY.png", "contentType" : "binary/octet-stream", "length" : 28297, "chunkSize" : 261120, "uploadDate" : ISODate("2015-03-19T02:31:10.069Z"), "aliases" : null, "metadata" : { "id" : "mkPQ6wnY" }, "md5" : "85944f92db940c6590fc8059db91ea16" }
  &gt;db.fs.files.find({"metadata": {"id": { $in: ["mkPQ6wnY","mJnB2DhK"]}}})
  &gt;
  </code></pre>
  
  <p>I can't find the difference on this query against the one in the <a href="http://docs.mongodb.org/manual/reference/operator/query/in/#use-the-in-operator-to-match-values" rel="nofollow">docs</a>, except that my <em>$in</em> comparison is in a second level, but I don't think that would be a problem for MongoDB.</p>
  
  <p>I guess is a very silly mistake, but I can't see it.</p>