順序付きのHash OrderedHash

$ script/console 
Loading development environment (Rails 2.3.2)
>> RUBY_VERSION
=> "1.8.7"
>> ActiveSupport::OrderedHash.new(:a => 1, :b => 2)
=> #<OrderedHash {}>
>> ActiveSupport::OrderedHash.new([[:a, 1], [:b, 2]])
=> #<OrderedHash {}>
>> hash = ActiveSupport::OrderedHash.new
=> #<OrderedHash {}>
>> hash[:a] = 1
=> 1
>> hash[:b] = 2
=> 2
>> hash
=> #<OrderedHash {:a=>1, :b=>2}>

Rails と Ruby の Ordered Hash について
HashとActiveSupport::OrderedHashのmergeの挙動が異なる
Rails 2.3 の ActiveSupport::OrderedHash がなんか変