class Class

C.descendants # => [B, A, D]
class D < C; end

C.descendants # => [B, A]
class A < B; end

C.descendants # => [B]
class B < C; end

C.descendants # => []
class C; end

Returns an array with all classes that are < than its receiver.

def descendants
  subclasses.concat(subclasses.flat_map(&:descendants))
end