order results from a mongoid relations according to how they are stored in the relationship array of ids

mongoid_monkey_patch.rb

# place this file in your initializers directory to have a nice to_ordered_a method to be able to retrieve an array ordered according to the order of the *_ids source.
# why doing this ? Because MongoDB allows to store array of objects. An array is not only a nice structure to store ids, it also implies that the ids are stored in a certain order. By default when you call relations in mongoid, the objects are returned in some order (certainly defined by the order of creation), not the order stores in your target array of ids.

module Mongoid #:nodoc:
  module Relations #:nodoc:
      class Many
        def to_ordered_a
          original_array_ids = base.send metadata.key.to_sym
          original_array_indexes = Hash[original_array_ids.each_with_index.to_a]
          target.sort_by{ |s| original_array_indexes[s.id] }
        end
      end
  end
end

module Mongoid #:nodoc:
  module Relations #:nodoc:
    module Targets
      class Enumerable
        def to_ordered_a
          original_array_ids = base.send metadata.key.to_sym
          original_array_indexes = Hash[original_array_ids.each_with_index.to_a]
          target.sort_by{ |s| original_array_indexes[s.id] }
        end
      end
    end
  end
end

module Mongoid #:nodoc:
  class Criteria
    def to_ordered_a
      original_array_ids = selector[:_id]["$in"]
      original_array_indexes = Hash[original_array_ids.each_with_index.to_a]
      entries.sort_by{ |s| original_array_indexes[s.id] }
    end
  end
end