validates_presence_of :name, :description
:if => Proc.new { |record| record.parent_id.blank? }

I realize this post is old now, but I thought I’d comment anyway

In my Thumbnail model I included belongs_to :person and in my Person model I had both has_one :photo and has_one :thumbnail. I got ride of belongs_to :person (in Thumbnail model) and got rid of has_one :thumbnail (in Person model) restarted the server and your example worked.

Given that I only have 1 Thumbnail per Photo I only need to do:
p.photo.thumbnails

I’m not sure why your photos aren’t showing thumbnails. Mine work fine. Maybe you could describe exactly what you mean by “does not work” ? Error? Empty set?

I do the same thing you’re trying to accomplish with your Person model, except mine is called User. I do something like

u = User.find 1
u.photos.first.thumbnails.first

using

class User
  has_many :photos
end

class Photo
  belongs_to :user
end

to give me the thumbnail. It sounds like you only have one Photo per Person, so you could probably do something like

p = Person.find 1
p.photo.thumbnails.first

Do a

class Person
  def thumbnail
    thumbnails.first
  end
end

to make your life a bit easier and allow

p = Person.find 1
p.photo.thumbnail

This is, of course, assuming you want only one Thumbnail per Photo.

In the last lines you show off the nice relationship between photo and thumbnails. So for a given thumbnail you can find its photo. I can’t seem to get it work as easily the other way arround: for a given photo I’d like to find its thumbnail.

for instance:
p = Photo.find :all
p.first.thumbnails

does not work!

As an added layer of complexity, my photos belong_to :person. So for a given person I can find their photo. However I can’t figure out to include the person_id (which connects a given photo with a person) into each thumbnail database entry (to connect a given thumbnail with a person).

Any suggestions?

This is exactly what I’ve been after for a while.

Thanks a million!