gistfile1.rb

class Instance < ActiveRecord::Base
  has_many :reports
  has_many :exams, primary_key: 'study_instance_uid', foreign_key: 'study_inst_uid', inverse_of: :instance
  has_many :patients, through: :exams

  before_save :set_reading_physician
 
  def reading_physician
    (set_reading_physician and save!) if reading_physician.nil?
    read_attribute(:reading_physician) || ''
  end
  
  private
 
  def set_reading_physician
    self.reading_physician ||= patient.reading_physician
  end
end