Look up all descendants of a class in Ruby

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

Alternatively (updated for ruby 1.9+):

ObjectSpace.each_object(YourRootClass.singleton_class)

Ruby 1.8 compatible way:

ObjectSpace.each_object(class<<YourRootClass;self;end)

Note that this won't work for modules. Also, YourRootClass will be included in the answer. You can use Array#- or another way to remove it.

Answer 2

You can require 'active_support/core_ext' and use the descendants method. Check out the doc, and give it a shot in IRB or pry. Can be used without Rails.

Answer 3

This method will return a multidimensional hash of all of an Object's descendants.

def descendants_mapper(klass)
  klass.subclasses.reduce({}){ |memo, subclass|
    memo[subclass] = descendants_mapper(subclass); memo
  }
end

{ MasterClass => descendants_mapper(MasterClass) }

Answer 4

Ruby Facets has Class#descendants,

require 'facets/class/descendants'

It also supports a generational distance parameter.

Answer 5

A simple version that give an array of all the descendants of a class:

def descendants(klass)
  all_classes = klass.subclasses
  (all_classes + all_classes.map { |c| descendants(c) }.reject(&:empty?)).flatten
end

Answer 6

Using descendants_tracker gem may help. The following example is copied from the gem's doc:

class Foo
  extend DescendantsTracker
end

class Bar < Foo
end

Foo.descendants # => [Bar]

This gem is used by the popular virtus gem, so I think it's pretty solid.