By default, if you don't use sort, the returned results always ordered by the order they inserted.

It is also true for MySQL. 

 
mysql> select * from category where short_name in ( 'mysql', 'cpp','oracle','stl');
+----+------------+-----------+-----------+----------+------+
| id | short_name | show_name | parent_id | keywords | desc |
+----+------------+-----------+-----------+----------+------+
| 11 | oracle     | Oracle    |         0 |          |      |
| 12 | mysql      | MySQL     |         0 |          |      |
| 22 | cpp        | C++       |         0 |          |      |
| 29 | stl        | STL       |        22 |          |      |
+----+------------+-----------+-----------+----------+------+
4 rows in set (0.00 sec)
 
mysql> select * from category where short_name in ( 'mysql', 'cpp','oracle','stl') order by short_name;
+----+------------+-----------+-----------+----------+------+
| id | short_name | show_name | parent_id | keywords | desc |
+----+------------+-----------+-----------+----------+------+
| 22 | cpp        | C++       |         0 |          |      |
| 12 | mysql      | MySQL     |         0 |          |      |
| 11 | oracle     | Oracle    |         0 |          |      |
| 29 | stl        | STL       |        22 |          |      |
+----+------------+-----------+-----------+----------+------+
4 rows in set (0.00 sec)
 

In MongoDB, use sort:

 
$result =  $collection->find(array('_id' => array('$in' => $ids)))->sort(array('insertdate' => -1));