def add(column_name, direction = nil)
      direction ||= 'ASC'
      direction = direction.to_s.upcase
      column = get_column(column_name)
      raise ArgumentError, "Could not find column #{column_name}" if column.nil?
      raise ArgumentError, 'Sorting direction unknown' unless %i[ASC DESC].include? direction.to_sym
      @clauses << [column, direction.untaint] if column.sortable?
      raise ArgumentError, "Can't mix :method- and :sql-based sorting" if mixed_sorting?
    end
def add(column_name, direction = nil)
      direction ||= 'ASC'
      direction = direction.to_s.upcase
      column = get_column(column_name)
      raise ArgumentError, "Could not find column #{column_name}" if column.nil?
      raise ArgumentError, 'Sorting direction unknown' unless %i[ASC DESC].include? direction.to_sym
      @clauses << [column, direction.untaint] if column.sortable?
      raise ArgumentError, "Can't mix :method- and :sql-based sorting" if mixed_sorting?
    end
add a clause to the sorting, assuming the column is sortable