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

def reading_physician
  self[:reading_physician] || patient.try(:reading_physician) || ''
end