sub.rb

class Subscription::Plan
    has_many :scan_frequencies
    def scan_frequencies_sorted
      scan_frequencies.sort do |a, b|
        a.frequency.times_per_year <=> b.frequency.times_per_year
      end
    end
end

class Subscription::ScanFrequency
   belongs_to :frequency
end

class Frequency
  validates_inclusion_of :times_per_year, :in => [1, 4, 12, 52, 365]
  has_many :scan_frequencies
end

# Example usages

scan_freq = plan.scan_frequencies_sorted.first

scan_freq.frequency.times_per_year
scan_freq.frequency.name

# would become
class Subscription::Plan
    has_many :scan_frequencies, :order => 'times_per_year'
end

class Subscription::ScanFrequency
  FREQUENCY_TO_NAME = {1 => 'Annually',
    4 => 'Quarterly',
    12 => 'Monthly',
    52 => 'Weekly',
    365 => 'Daily'
  }
  validates_inclusion_of :times_per_year, :in => FREQUENCY_TO_NAME.keys
  def frequency_name
    FREQUENCY_TO_NAME[self.times_per_year]
  end
end
# Example usages

# no need to sort, already done
scan_freq = plan.scan_frequencies.first


scan_freq.times_per_year
scan_freq.frequency_name