php - mongodb->在文档中排序

Question

每个文档我都有这个结构：

{
"_id": NumberInt(781),
"link_id": {
 "11617": {
   "thumb_position": NumberInt(654),
   "last_scan": NumberInt(1362671892),
   "row_numb": NumberInt(0),
   "clicks": NumberInt(0)
},
 "11623": {
   "thumb_position": NumberInt(562),
   "last_scan": NumberInt(1362671892),
   "row_numb": NumberInt(0),
   "clicks": NumberInt(0)
},
 "11689": {
   "thumb_position": NumberInt(93),
   "last_scan": NumberInt(1362671892),
   "row_numb": NumberInt(0),
   "clicks": NumberInt(0)
}, {...}
}

事实是我想通过 thumb_position 订购。我正在使用： $res = $collection->find(array("_id" => intval(781)))->sort(array('link_id.thumb_position' => 1))

它不工作。我试过 ->sort(array('link_id.$.thumb_position' => 1)) 或 ->sort(array('thumb_position' => 1)) 和相应的 $collection->ensureIndex。没运气。我见过一些排序的例子，但它们都与排序文档有关，文档内部没有排序。有任何想法吗？！多谢！