Rails - Aug 2006 - Single Table Inheritance with self-referential join

Please tell me if this train of thinking makes sense, I need help on 
solving this association model problem:

I am doing an app that creates and manages stereoscopic images, one 
stereoscopic image is formed by two photos, say photo1 and photo2

The user uploads photo1 and then photo2 and gets a preview if he likes 
the preview he/she authorizes it and gets a final stereoscopic image

All this images will be managed via file_column/RMagic plugins

When the user uploads photo1 and photo2 this are placed into an order, 
(as when you place an order of fries...)
So there is an Order model that has_one of each:

Photo1 (high-res) belongs_to :order
Photo2 (high-res) belongs_to :order
Preview (low-res) belongs_to :order
Final  (high-res) belongs_to :order

Each of Photo1, Photo2, Preview and Final models, need to have 
additional user/server-information fields.

I am thinking of implementing this as a "Single Table Inheritance"
with
self-referential join modeled like this:


create_table :stereos, :force => true do |t|

t.column :type, :string

# common attributes
t.column :photo_name, :string
t.column :created_at, :datetime
t.column :updated_at, :datetime
t.column :order_id,   :integer


# attributes for type=Photo
t.column :date_taken, :datetime
t.column :pair_id, :integer

# attributes for type=Preview
t.column :authorized_by_id, :integer
t.column :photo1_id, :integer
t.column :photo2_id, :integer

# attributes for type=Final
t.column :photo1_id, :integer
t.column :photo2_id, :integer
end



Now for the hierarchy of model objects:

class Stereo < ActiveRecord::Base
end

class Photo < Stereo
end

class Preview < Stereo
end

class Final < Stereo
end


So the questions are:

1.- Is this a correct way to solve this relationships modeling problem, 
will this work or is there a flow?

2.- photo1 and photo2 allways have to be associated in pairs, am i doing 
this correctly via the pair_id field?

3.- preview is the product of merging photo1 and photo2 so i need to 
make shure that each preview points to it''s parent via photo1_id and 
photo2_id

4.- final is the product of merging photo1 and photo2 so i need to make 
shure that each final points to it''s parent via photo1_id and photo2_id

5.- How should I manage field validations, where should I put them?

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A stereo image is two almost identical images (one slightly offset)
placed side-by-side such that when viewed properly will produce a 3D
effect.  Therefore you might be better off with this hierarchy because
Photo, Preview, and Final are all Images regardless of their source.
Then have a separate Stereo class be the parent relating the 3-types of
images.

class Image < ActiveRecord::Base
  belongs_to :stereo

class Photo < Image

class Preview < Image

class Final < Image


class Stereo < ActiveRecord::Base
  has_many :images


create_table :images, :force => true do |t|

t.column :type, :string

# magic attributes
t.column :type        :string  # required to activate STI
t.column :created_at, :datetime

# common attributes
t.column :stereo_id   :integer # the parent
t.column :path        :string  # file has to be somewhere
t.column :photo_name, :string

# attributes for type=Photo
t.column :date_taken, :datetime # irrelevant to the task?
t.column :is_left     :boolean #

# attributes for type=Preview

# attributes for type=Final
end


create_table :stereos, :force => true do |t|

# magic attributes
t.column :created_at, :datetime
t.column :updated_at, :datetime

# common attributes
t.column :order_id,   :integer  # An Order has many Stereos

# a user actually authorizes a stereo to be finalized
t.column :authorized_by_id, :integer

end


Don''t forget you have to define each class in it''s own model
file for
it to work. Also take my advice with a grain of salt as I''ve only been
at this for a week and only found out about STI 2 days ago. :)


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