SEO friendly topic url

topic.rb

class Topic < ActiveRecord::Base
  
  (...)

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

  (...)

end

topics_controller.rb

class TopicsController < ApplicationController
  
  def index
    @topics = Topic.order('mentions desc')
  end
  
  def show
    # find calls to_i on params[:id]
    # so the string part is removed
    @topic = Topic.find(params[:id])
  end
  
end