has_many :photo_relationships
 has_many :attributes, :through => :photo_relationships, :source => :related_attribute, :source_type => "Attribute"

belongs_to :photo #photo_id
 belongs_to :related_attribute, :polymorphic => true

has_many :photo_relationships, :as => :related_attribute, :dependent => :destroy
 has_many :photos, :through => :photo_relationships

photo = Photo.first
 photo.attributes.first.rating