I've added a type check to the processing of the :fields options to skip the addition of :_type to the fields list if :field is a hash. This should maintain the current behavior as well as provide support for the $slice operator, which requires that :fields be a hash, not an array.

http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields#RetrievingaSubsetofFields-RetrievingaSubrangeofArrayElements

It is now possible to manipulate the range of subfields in a document. Compare the following:
Service.last.service_logs.count
=> 66

Service.all.extras(:fields => {:service_logs => {"$slice" => 10}}).last.service_logs.count
=> 10