add_child

# File activerecord/lib/active_record/acts/nested_set.rb, line 141
        def add_child( child )     
          self.reload
          child.reload

          if child.root?
            raise "Adding sub-tree isn\'t currently supported"
          else
            if ( (self[left_col_name] == nil) || (self[right_col_name] == nil) )
              # Looks like we're now the root node!  Woo
              self[left_col_name] = 1
              self[right_col_name] = 4
              
              # What do to do about validation?
              return nil unless self.save
              
              child[parent_column] = self.id
              child[left_col_name] = 2
              child[right_col_name]= 3
              return child.save
            else
              # OK, we need to add and shift everything else to the right
              child[parent_column] = self.id
              right_bound = self[right_col_name]
              child[left_col_name] = right_bound
              child[right_col_name] = right_bound + 1
              self[right_col_name] += 2
              self.class.base_class.transaction {
                self.class.base_class.update_all( "#{left_col_name} = (#{left_col_name} + 2)",  "#{scope_condition} AND #{left_col_name} >= #{right_bound}" )
                self.class.base_class.update_all( "#{right_col_name} = (#{right_col_name} + 2)",  "#{scope_condition} AND #{right_col_name} >= #{right_bound}" )
                self.save
                child.save
              }
            end
          end                                   
        end