https://github.com/localshred/protobuf/issues/58

test--proto--addressbook_ext.pb.rb

### Generated by rprotoc. DO NOT EDIT!
### <proto file: test/proto/addressbook_ext.proto>
# import "test/proto/addressbook_base.proto";
# package tutorial_ext;
# 
# extend Person {
#   optional int32 age = 100;
# }

require 'protobuf/message/message'
require 'protobuf/message/enum'
require 'protobuf/message/extend'

require 'test/proto/addressbook_base.pb'
module TutorialExt
  class Person < ::Protobuf::Message
    defined_in __FILE__
    optional :int32, :age, 100, :extension => true
  end
end

test.sh

cd ~/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/protobuf-2.6.4
mkdir tmp
rprotoc --ruby_out=tmp test/proto/addressbook*.proto 
diff test/proto/addressbook_ext.pb.rb tmp/test/proto/addressbook_ext.pb.rb 

tmp--test--proto--addressbook_ext.pb.rb

##
# This file is auto-generated. DO NOT EDIT!
#

##
# Imports
#
require 'test/proto/addressbook_base.pb'

module Tutorial_Ext
  
  
end