# File lib/groupdate/scopes.rb, line 14
    def group_by_period(period, field, options = {})
      # to_sym is unsafe on user input, so convert to strings
      permitted_periods = ((options[:permit] || Groupdate::PERIODS).map(&:to_sym) & Groupdate::PERIODS).map(&:to_s)
      if permitted_periods.include?(period.to_s)
        send("group_by_#{period}", field, options)
      else
        raise ArgumentError, "Unpermitted period"
      end
    end