:s3_host_name can be a proc

lib/paperclip/storage/s3.rb

@@ -172,7 +172,10 @@ module Paperclip
       end
       end
 
 
       def s3_host_name
       def s3_host_name
-        @options[:s3_host_name] || s3_credentials[:s3_host_name] || "s3.amazonaws.com"
+        host_name = @options[:s3_host_name]
+        host_name = host_name.call(self) if host_name.is_a?(Proc)
+
+        host_name || s3_credentials[:s3_host_name] || "s3.amazonaws.com"
       end
       end
 
 
       def s3_host_alias
       def s3_host_alias

test/storage/s3_test.rb

@@ -238,6 +238,26 @@ class S3Test < Test::Unit::TestCase
     end
     end
   end
   end
 
 
+  context "dynamic s3_host_name" do
+    setup do
+      rebuild_model :storage => :s3,
+                    :s3_credentials => {},
+                    :bucket => "bucket",
+                    :path => ":attachment/:basename.:extension",
+                    :s3_host_name => lambda {|a| a.instance.value }
+      @dummy = Dummy.new
+      class << @dummy
+        attr_accessor :value
+      end
+      @dummy.avatar = StringIO.new(".")
+    end
+
+    should "use s3_host_name as a proc if available" do
+      @dummy.value = "s3.something.com"
+      assert_equal "http://s3.something.com/bucket/avatars/stringio.txt", @dummy.avatar.url(:original, :timestamp => false)
+    end
+  end
+
   context "An attachment that uses S3 for storage and has styles that return different file types" do
   context "An attachment that uses S3 for storage and has styles that return different file types" do
     setup do
     setup do
       rebuild_model :styles  => { :large => ['500x500#', :jpg] },
       rebuild_model :styles  => { :large => ['500x500#', :jpg] },