allow proc for fog_credentials

lib/paperclip/storage/fog.rb

@@ -163,8 +163,10 @@ module Paperclip
           YAML::load(ERB.new(File.read(creds)).result)
         when Hash
           creds
+        when Proc
+          creds.call(self)
         else
-          raise ArgumentError, "Credentials are not a path, file, or hash."
+          raise ArgumentError, "Credentials are not a path, file, hash or proc."
         end
       end
 

test/storage/fog_test.rb

@@ -249,6 +249,25 @@ class FogTest < Test::Unit::TestCase
           assert_match /http:\/\/dynamicfoghost\.com/, @dummy.avatar.url
         end
       end
+
+      context "with a proc for the fog_credentials evaluating a model method" do
+        setup do
+          @dynamic_fog_credentials = {
+            :provider               => 'AWS',
+            :aws_access_key_id      => 'DYNAMIC_ID',
+            :aws_secret_access_key  => 'DYNAMIC_SECRET'
+          }
+          rebuild_model(@options.merge(:fog_credentials => lambda { |attachment| attachment.instance.fog_credentials }))
+          @dummy = Dummy.new
+          @dummy.stubs(:fog_credentials).returns(@dynamic_fog_credentials)
+          @dummy.avatar = @file
+          @dummy.save
+        end
+
+        should "provide a public url" do
+          assert_equal @dummy.avatar.fog_credentials, @dynamic_fog_credentials
+        end
+      end
     end
 
   end