How to organize a tree model that is part of another model?

Question

Ruby on Rails

class User < ActiveRecord::Base
  has_and_belongs_to :folders
end

# ...

# migration
class UsersFolders < ActiveRecord::Migration
  def change
    create_table :folders_users, id: false do |t|
      t.belongs_to :user
      t.belongs_to :folder
    end
  end
end

# ...

class Folder < ActiveRecord::Base
  has_ancestry 
  has_and_belongs_to: users
end

The main problem is that root can refer to users, but its children are nil for users.
Thank you.

Answer 1

The question is not entirely clear