app-model-document.rb

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?
      return @tree
    else
      @tree << who.parent
      get_ancestors(who.parent)
    end
  end

  def ancestors
    @ancestors ||= Document.get_ancestors(self)
  end

end

console

    # Recursion will generate array of parents parents
    d = Document.last
    d.ancestors.collect(&:id)
    # => [570, 569, 568] 

db-migration-create_documents.rb

class CreateDocuments < ActiveRecord::Migration
  def change
    create_table :documents do |t|
      t.integer :parent_id
      t.string  :file
      t.timestamps
    end
  end
end