allow proc for fog_host

lib/paperclip/storage/fog.rb

@@ -126,7 +126,12 @@ module Paperclip
 
       def public_url(style = default_style)
         if @options[:fog_host]
-          host = (@options[:fog_host] =~ /%d/) ? @options[:fog_host] % (path(style).hash % 4) : @options[:fog_host]
+          host = if @options[:fog_host].is_a?(Proc)
+            @options[:fog_host].call(self)
+          else
+            (@options[:fog_host] =~ /%d/) ? @options[:fog_host] % (path(style).hash % 4) : @options[:fog_host]
+          end
+          
           "#{host}/#{path(style)}"
         else
           if fog_credentials[:provider] == 'AWS'

test/storage/fog_test.rb

@@ -235,6 +235,20 @@ class FogTest < Test::Unit::TestCase
         end
 
       end
+
+      context "with a proc for the fog_host evaluating a model method" do
+        setup do
+          rebuild_model(@options.merge(:fog_host => lambda { |attachment| attachment.instance.fog_host }))
+          @dummy = Dummy.new
+          @dummy.stubs(:fog_host).returns('http://dynamicfoghost.com')
+          @dummy.avatar = @file
+          @dummy.save
+        end
+
+        should "provide a public url" do
+          assert_match /http:\/\/dynamicfoghost\.com/, @dummy.avatar.url
+        end
+      end
     end
 
   end