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. 
