    # File lib/active_record/acts/nested_set.rb, line 199
            def before_destroy
              return if self[right_col_name].nil? || self[left_col_name].nil?
              dif = self[right_col_name] - self[left_col_name] + 1

              self.class.base_class.transaction {
                self.class.base_class.delete_all( "#{scope_condition} and #{left_col_name} > #{self[left_col_name]} and #{right_col_name} < #{self[right_col_name]}" )
                self.class.base_class.update_all( "#{left_col_name} = (#{left_col_name} - #{dif})",  "#{scope_condition} AND #{left_col_name} >= #{self[right_col_name]}" )
                self.class.base_class.update_all( "#{right_col_name} = (#{right_col_name} - #{dif} )",  "#{scope_condition} AND #{right_col_name} >= #{self[right_col_name]}" )
              }
            end