Untitled

Ordering by a two fields, one being created_at, In Rails
Model.order('popularity ASC')

Model.order('popularity-integerize(created_at) ASC')

class Model < ActiveRecord::Base
  def decaying_popularity
    popularity + created_at.to_i # created_at is larger for newer creations.
  end                            # you'll obviously need to edit the formula you use.

  def self.order_by_decaying_popularity
    models = self.all
    models.sort {|a,b| a.decaying_popularity <=> b.decaying_popularity }
  end
end

Model.order_by_decaying_popularity

Model.where(:author_id => 1).order_by_decaying_popularity