object_space_identity_map.rb

# Instances must supply a Klass.load method which is used to load the object 
# in case it is not available in object space - for example from some data store.
# Also objects must provide an id method which is used as the key
# 
module ObjectSpaceIdentityMap
  def [](id)
    obj = nil
    ObjectSpace.each_object(self) do |o|
      obj = o if o.id == id
    end
    obj = load(id) unless obj
    return obj
  end
end

class Foo
  extend ObjectSpaceIdentityMap
  
  attr_reader :id
  
  def initialize(id)
    @id = id
  end
  
  # This is a really trivial load - it would probably load from some datastore
  def self.load(id)
    new(id)
  end
  
  def to_s
    "Foo object with name #{@name} and object_id #{object_id}"
  end
end

puts Foo['bob']
puts Foo['charles']
puts Foo['bob'] # Should have the same object_id as first bob