Question

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.)

Answer 1

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