gistfile1.rb

class Event < ActiveRecord::Base

  has_many :artists

end

class Artist < ActiveRecord::Base

  def self.make_me_an_artist
    create! do |artist|
      artist.cached_event_time = artist.event.time
    end
  end

end

event = Event.create(time: "3pm")
event.artists.make_me_an_artist

event.artists.first.cached_event_time #=> "3pm"