Variety of improvements and additions:

[selectricity] / app / models / vote.rb

class Vote < ActiveRecord::Base
  belongs_to :voter
  has_many :rankings
  
  def initialize
    super
    @votes = []
  end
  
  def votestring=(string="")
    rel_votes = string.split("").collect { |vote| vote.to_i }
    
    # covert relative orders to absolute candidate ids
    candidate_ids = voter.election.candidates.sort
    candidate_ids.collect! { |candidate| candidate.id.to_i }
   
    rel_votes.collect! { |vote| candidate_ids[ vote - 1 ] }
    @votes = rel_votes
  end

  def save 
    rankings.each { destroy } unless rankings.empty?
    @votes.each_with_index do |candidate, index| 
      ranking = Ranking.new
      ranking.rank = index + 1
      ranking.candidate =  Candidate.find(candidate)
      self.rankings << ranking
    end
      
    super
  end

  def destroy
    rankings.each { destroy }
    super
  end

end