equivalent

class Document < ActiveRecord::Base

  belongs_to :parent, class_name: 'Document'

  def self.get_ancestors(who)
    @tree ||= []
    # @tree is instance variable of Document class object not document instance object
    # so: Document.get_instance_variable('@tree')

    if who.parent.nil?

mbostock

license: gpl-3.0
redirect: https://observablehq.com/@d3/d3-collapsible-tree

yuya-takeyama

module BinaryTree
  class Node
    attr_reader :word, :count, :left, :right

    include Enumerable

    def initialize(word)
      @word, @count = word, 1
    end