gistfile1.rb

class Photo < ActiveRecord::Base
  scope :enabled, -> { where(photo: { enabled: true }) }
end

class Person < ActiveRecord::Base
  has_one :display_photo, -> {
    self.select_values = ["DISTINCT ON(photos.person_id) photos.*"]
    instance_exec(&Photo.method(:enabled)).order('photos.person_id')
    }, class_name: "Photo"
end