rails_3_scope_options.rb

scope :latest_by_hashtag, lambda {|hashtag| {
  :where => { :hashtags => hashtag },
  :order_by => :tweeted_at.desc,
  :limit => 20 }
}

# OR #

module MyModelModules
  module Scopes
    extend ActiveSupport::Concern
    included do
      scope :latest_by_hashtag, lambda {|hashtag| {
        :where => { :hashtags => hashtag },
        :order_by => :tweeted_at.desc,
        :limit => 20 }
      }
    end
  end
end

class MyModel
  include Mongoid::Document
  include MyModelModules::Scopes
  # ...
end