tsort_example.rb

# Ruby's TSort requires that you implement 
# tsort_each_node and tsort_each_child. I 
# extend Array so that it knows how to 
# TSort instances of Project, which has a 
# child_project_id pointing to another 
# Project.

require 'tsort'

class Array
  include TSort

  alias tsort_each_node each

  def tsort_each_child(node, &block)
    if node.is_a?(Project)
      [node.child_project].each(&block) if node.child_project?
    else
      node.each(&block) if node
    end
  end
end

# Then I can topologically sort a user's projects like so:
sorted_projects = current_user.projects.tsort