app/models/product.rb

   1 class Product < ActiveRecord::Base
   2   validates_presence_of :title, :description, :image_url
   3   validates_numericality_of :price
   4   validate :price_must_be_at_least_a_penny
   5   validates_uniqueness_of :title
   6   validates_format_of :image_url,
   7                       :with    => %r{\.(gif|jpg|png)$}i,
   8                       :message => "must be a URL for GIF, JPG, or PNG image."
   9
  10 protected
  11   def price_must_be_at_least_a_penny
  12     errors.add(:price, 'should be at least 0.01') if price.nil? || price < 0.01
  13   end
  14 end