請參閱Still ignoring that this is an array, we can use dot notation to query the subobject ....
http://php.net/manual/en/mongo.queries.php

使用array( "element" => array( ... ) )方法無效,
正確的用法應該是類似json的呼叫子物件方式
如$field = array( "element" => 1 , "element2.subElement" => 1 )