# File lib/ancestry/class_methods.rb, line 151
    def build_ancestry_from_parent_ids! parent_id = nil, ancestry = nil
      self.base_class.send(:with_exclusive_scope) do 
        self.base_class.find_each(:conditions => {:parent_id => parent_id}) do |node|
          node.without_ancestry_callbacks do
            node.update_attribute ancestry_column, ancestry
          end
          build_ancestry_from_parent_ids! node.id, if ancestry.nil? then "#{node.id}" else "#{ancestry}/#{node.id}" end
        end
      end
    end