# app/models/admin_user.rb
class AdminUser
  include Mongoid::Document
  
  #...
  
  def self.serialize_from_session(key, salt)
    record = to_adapter.get(key[0]["$oid"])
    record if record && record.authenticatable_salt == salt
  end
end

# config/initializers/ext/bson.rb
module BSON
  class ObjectId
    def to_json(*args)
      return to_s if args.blank?
      args[0].generate(to_s)
    end

    alias :as_json :to_json
  end
end

module Mongoid
  module Document
    def to_key
      (persisted? || destroyed?) ? [ id.to_s ] : nil
    end
  end
end