gistfile1.rb

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

# Polymorphic association example
class Person < ActiveRecord::Base
  has_many :photos, as: :photo_parent
  has_one :display_photo, -> {
    fields = "photos.photo_parent_id, photos.photo_parent_type"
    self.select_values = ["DISTINCT ON(#{fields}) photos.*"]
    order(fields)
    }, as: :photo_parent, class_name: "Photo"
end

people = Person.all.includes(:display_photo)
people.each { |person| puts person.display_photo.id }