ruby
module Mongoid
  class Criteria

    def project(* relationship)
      if projected_entries.any?
        symbol = Array(relationship).last # eg. :artist
        klass = symbol.to_s.camelize.constantize # eg. Artist
        accessors = Array(relationship)[0..-2] # eg. [ :artwork ]

        parent = self.klass
        accessors.each do |accessor|
          parent = parent.relations[accessor.to_s].klass
        end

        # examine the relationship
        relation = parent.relations[symbol.to_s]

        # fetch the IDs
        target_ids = projected_entries.map do |entry|
          # apply any deep accessors to find the relationship parent
          accessors.each do |accessor|
            entry = entry.send(accessor)
          end
          entry[relation.foreign_key] if entry # id stored in the model
        end.compact.uniq

        # fetch the instances
        target_instances = Hash[klass.find(target_ids).map do |target_instance|
          [ target_instance.id, target_instance ]
        end] if target_ids.any?

        # project instances
        projected_entries.each do |entry|
          # apply any deep accessors
          accessors.each do |accessor|
            entry = entry.send(accessor)
          end
          # identity of the target instance
          target_instance = target_instances[entry[relation.foreign_key]] if entry
          entry.send("#{symbol}=", target_instance) if entry && target_instance
        end if target_instances
      end
      self
    end

    def projected_entries
      @projected_entries ||= entries
    end
  end
end

 ruby
Gadget.limit(5).all.project(:widget).projected_entries