I had this issue recently when trying to sort items in an EmbeddedDocument. Say you have a "Post" Document and a "Comment" EmbeddedDocument. If you want to sort the comments by the created_at field (which is pretty likely), it isn't as easy as you would think, using MongoMapper (at the moment anyway).
However, there is a nice clean alternative that can be used to achieve the same results. Simply use the "sort_by" method like so, passing in a symbol of the field to sort as a proc:-
...and for descending order:-
Here is the original google group thread.