Add some more documentation for passing symbol to :url, :default_url, :path

lib/paperclip.rb

@@ -291,6 +291,20 @@ module Paperclip
     #   choices are :filesystem and :s3. The default is :filesystem. Make sure you read the
     #   documentation for Paperclip::Storage::Filesystem and Paperclip::Storage::S3
     #   for backend-specific options.
+    #
+    # It's also possible for you to dynamicly define your interpolation string for :url,
+    # :default_url, and :path in your model by passing a method name as a symbol as a argument
+    # for your has_attached_file definition:
+    #
+    #   class Person
+    #     has_attached_file :avatar, :default_url => :default_url_by_gender
+    #
+    #     private
+    #
+    #     def default_url_by_gender
+    #       "/assets/avatars/default_#{gender}.png"
+    #     end
+    #   end
     def has_attached_file name, options = {}
       include InstanceMethods
 

lib/paperclip/interpolations.rb

@@ -25,7 +25,8 @@ module Paperclip
 
     # Perform the actual interpolation. Takes the pattern to interpolate
     # and the arguments to pass, which are the attachment and style name.
-    # You can pass a :symbol as pattern assigning some method in your model class.
+    # You can pass a method name on your record as a symbol, which should turn
+    # an interpolation pattern for Paperclip to use.
     def self.interpolate pattern, *args
       pattern = args.first.instance.send(pattern) if pattern.kind_of? Symbol
       all.reverse.inject( pattern.dup ) do |result, tag|