gistfile1.rb

module Subclasses
  # return a list of the subclasses of a class
  def subclasses(direct = false)
    classes = []
    if direct
      ObjectSpace.each_object(Class) do |c|
        next unless c.superclass == self
        classes << c
      end
    else
      ObjectSpace.each_object(Class) do |c|
        next unless c.ancestors.include?(self) and (c != self)
        classes << c
      end
    end
    classes
  end
end
 
class Parent
  def self.runnable?
    false
  end
end

class Child < Parent
  def self.runnable?
    true
  end
end

class Grandchild < Child
  def self.runnable?
    false
  end
end

Object.send(:include, Subclasses)
puts Parent.subclasses.select { |c| c.runnable? }.inspect