proc_patterns_for_paperclip.rb

class Product < ActiveRecord::Base

  has_many :images, :class_name => "ProductImage"

end

class ProductImage < ActiveRecord::Base

  belongs_to :product

  has_attached_file :image,
    :storage => :filesystem,
    :path    => ":rails_root/public/images/products/:attachment/:id/:style/:basename.:extension",
    :url     => "/images/products/:attachment/:id/:style/:basename.:extension",
    
    # Because the :styles key can take a Proc object as an argument, we can specify the image
    # styles dynamically based on the image's label.
    :styles  => Proc.new {|this|
      defionition = ProductImageDefinitions.detect {|spec| spec[:label] == this.instance.label}
      definition[:styles].merge(:thumb => '200x200>')
    }

end