tracking subclasses

register_instances.rb

class Base
  def self.register_instances
    @@instances ||= {}
    @@instances[self] || = []
    @@subclass_instances ||= {}
    @@subclass_instances[self] || = []
  end

  register_instances

  def initialize
    @@instances[self.class].push self
    @@subclass_instances.keys.each |klass|     
      @@subclass_instances[klass].push self if self.is_a? klass 
    end   
  end

  def self.all(options = {})
     options = {
       subclasses: true, # For backwards compatibility.
     }.merge! options

     options[:subclasses] ? @@subclass_instances[self] : @@instances[self]
  end
end

class Sub < Base
  register_instances
end