OrderedHash

is now a subclass of Hash that preserves order (or _is_ a Hash if running Ruby 1.9 or greater)

You might not realize it preserves order because it delegates inspect to its super-class, Hash, which doesn’t preserve order. But you will see that order is preserved if you iterate or use the keys or values methods:

  >> names = ['Amy Irving', 'Jane Doe', 'John Doe', 'John Updike', 'Susan Anthony']
  >> ordered = names.group_by { |name| name.split.first }
  => #<OrderedHash {"John"=>["John Doe", "John Updike"], "Amy"=>["Amy Irving"], "Susan"=>["Susan Anthony"], "Jane"=>["Jane Doe"]}>

  >> ordered.keys
  => ["Amy", "Jane", "John", "Susan"]

  >> ordered.each { |first, full| puts first; full.each { |name| puts "  "+name } }
  Amy
    Amy Irving
  Jane
    Jane Doe
  John
    John Doe
    John Updike
  Susan
    Susan Anthony