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ActiveSupport::OrderedHash implements a hash that preserves insertion order.

oh = ActiveSupport::OrderedHash.new
oh[:a] = 1
oh[:b] = 2
oh.keys # => [:a, :b], this order is guaranteed

Also, maps the omap feature for YAML files (See http://yaml.org/type/omap.html) to support ordered items when loading from yaml.

ActiveSupport::OrderedHash is namespaced to prevent conflicts with other implementations.

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April 8, 2009 - (v2.3.2)
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Superclass of OrderedHash

Note that in Rails 2.3, OrderedHash changed from being a subclass of Array to a subclass of Hash. This is contrary to what the documentation says above.

October 26, 2012 - (v1.2.0 - v3.2.8)
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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"]}>

# (note that the inspect above is in undefined order)

>> ordered.keys                          # will be ordered properly
=> ["Amy", "Jane", "John", "Susan"]

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