AR call back with conditional

topic.rb

class Topic < ActiveRecord::Base
  
  before_create :set_start_trending
  before_save :set_hot_topic, :if => Proc.new { |topic| topic.mentions >= HOT_TOPIC_MENTIONS }

  scope :trending_top, lambda { |num| where("started_trending > ?", 1.day.ago).order('mentions desc').limit(num) }

  HOT_TOPIC_MENTIONS = 1000

  def to_param
    "#{id}-#{name.parameterize}"
  end
  
  private

    def set_start_trending
      self.started_trending = Time.now
    end
    
    def set_hot_topic
      self.hot_topic = true #if self.mentions > HOT_TOPIC_MENTIONS
    end

end