new_validations.rb

  validates :title, :description, :image_url, :presence => true 
  validates :price, :numericality => { :greater_than => 0 }
  validates :title, :uniqueness => true
  validates :image_url, 
            :format => { :with  => %r{\.(gif|jpg|png)$}i, 
                         :message => 'must be a URL for GIF, JPG or PNG image.' }