Look up all descendants of a class in Ruby

I can easily ascend the class hierarchy in Ruby:

String.ancestors     # [String, Enumerable, Comparable, Object, Kernel]
Enumerable.ancestors # [Enumerable]
Comparable.ancestors # [Comparable]
Object.ancestors     # [Object, Kernel]
Kernel.ancestors     # [Kernel]

Is there any way to descend the hierarchy as well? I'd like to do this

Animal.descendants      # [Dog, Cat, Human, ...]
Dog.descendants         # [Labrador, GreatDane, Airedale, ...]
Enumerable.descendants  # [String, Array, ...]

but there doesn't seem to be a descendants method.

(This question comes up because I want to find all the models in a Rails application that descend from a base class and list them; I have a controller that can work with any such model and I'd like to be able to add new models without having to modify the controller.)

Here is an example:

class Parent
  def self.descendants
    ObjectSpace.each_object(Class).select { |klass| klass < self }
  end
end

class Child < Parent
end

class GrandChild < Child
end

puts Parent.descendants
puts Child.descendants

puts Parent.descendants gives you:

GrandChild
Child

puts Child.descendants gives you:

GrandChild

If you use Rails >= 3, you have two options in place. Use .descendants if you want more than one level depth of children classes, or use .subclasses for the first level of child classes.

Example:

class Animal
end

class Mammal < Animal
end

class Dog < Mammal
end

class Fish < Animal
end

Animal.subclasses #=> [Mammal, Fish] 
Animal.descendants  #=> [Dog, Mammal, Fish]

Ruby 1.9 (or 1.8.7) with nifty chained iterators:

#!/usr/bin/env ruby1.9

class Class
  def descendants
    ObjectSpace.each_object(::Class).select {|klass| klass < self }
  end
end

Ruby pre-1.8.7:

#!/usr/bin/env ruby

class Class
  def descendants
    result = []
    ObjectSpace.each_object(::Class) {|klass| result << klass if klass < self }
    result
  end
end

Use it like so:

#!/usr/bin/env ruby

p Animal.descendants

Override the class method named inherited. This method would be passed the subclass when it is created which you can track.

Look up all descendants of a class in Ruby

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