image.rb

class Image < ActiveRecord::Base
 before_create, :set_width_and_height
 
 has_attached_file :img,
                   :url => "/system/images/:attachment/:id/:basename.:extension",
                   :path => "#{Rails.root}/public/system/images/:attachment/:id/:basename.:extension"

 def set_width_and_height
   # this next line is the magic line 
   geo = Paperclip::Geometry.from_file(img.to_file(:original))
   self.width = geo.width 
   self.height = geo.height
 end

end