topic.rb

  before_update :update_ranking

  def self.calculate_ranking(score, created_at)
    order = Math.log10(([score.abs,1].max))

    if score > 0
      sign = 1
    elsif score < 0
      sign = -1
    else
      sign = 0
    end

    seconds = created_at - DateTime.new(1970,1,1)
    long_num = order + sign * seconds / 45000
    (long_num * 10**7).round.to_f / (10**7)
  end

  def update_ranking
    self.ranking = Topic.calculate_ranking(self.pushes_count, self.created_at)
  end