example.rb

class Collection
  class Instance
    def initialize(id)
      @id = id
    end

    def say
      "I am #{@id}"
    end
  end

  def initialize(ary)
    @ary = ary
  end

  def inspect
    @ary
  end

  def [] id
    raise "Array does not contain #{id}" unless @ary.include?(id)
    Instance.new id
  end
end