Initializer to be able to set an updated_by on paper trailed models. Giving it an ActiveRecord object will store whodunnit as "ModelName::Id". Calling updated_by will return the ActiveRecord object. This is also applied to the Version model.

paper_trail_updated_by.rb

module PaperTrail
  module Model
    # Creates and updated_by attributes on all paper trailed models. If given
    # an ActiveRecord instance, PaperTrail.whodunnit will be set as
    # "ModelName::Id". Calling updated_by will return the ActiveRecord object.
    module InstanceMethods
      def updated_by=(updater)
        PaperTrail.whodunnit = PaperTrail::UpdatedByParser.to_s(updater)
      end

      def updated_by
        PaperTrail::UpdatedByParser.from_s(PaperTrail.whodunnit)
      end
    end
  end

  # Added an updated_by attribute to all Version records. Returns an
  # ActiveRecord object if whodunnit is formated as "ModelName::Id" and the
  # record can be found. Else, it will return the string.
  class Version < ::ActiveRecord::Base
    def updated_by
      PaperTrail::UpdatedByParser.from_s(whodunnit)
    end
  end

  # Helper class that will take a formated whodunnit string and return an
  # ActiveRecord object. It will also can take an ActiveRecord object and
  # convert it to "ModelName::Id" format for storate in the verions table.
  class UpdatedByParser
    def self.to_s(updater)
      return updater unless updater.is_a?(ActiveRecord::Base)
      return nil if updater.new_record?
      "#{updater.class.name}::#{updater.id}"
    end

    def self.from_s(updater)
      return updater unless updater.respond_to?(:split)
      parts = updater.split("::")
      obj_id = parts.pop
      parts.join.constantize.find(obj_id)
    rescue ActiveRecord::RecordNotFound,NameError
      updater
    end
  end
end