How to precisely control your queries with MongoMapper

I have an index like this in my database (for my table entries): {"created_at": -1, "search_id": 1, "services":1}

I can make use of that index if I perform a search in the Mongo console lik开发者_JAVA百科e this:

 db.entries.find({'search_id':1, 'services':2}).sort({'created_at': 
-1})

(if I perform an explain() on that query I can see that it's using the index).

However I can't replicate that behavior with MongoMapper. I've tried with a search like this:

Entry.where(:search_id => 1, :services => 2).order(['created_at', 
'descending'])

I've altered the order and tried everything, but when I perform an explain on it I get that it's using a BasicCursor (that is, no index) and if I do a "criteria" on that query I can see that it's not even trying to order the results in the search: I guess they are ordered afterwards once the results are back.

Is there a way to control your queries more precisely so you can make use of an index in cases like these?

My bad guys, I was using the order function incorrectly as this time it expects a symbol rather than an array:

Entry.where(:search_id => 1, :services => 2).order(:created_at.desc)