Use callbacks to make your controller concise

with_callback.rb

# More readable controller where "hot topic" logic delegate to model callback 
class TopicsController < ApplicationController
  def create
    @topic = Topic.new(params[:topic])
    if @topic.save
      redirect_to @topic, :notice => 'Successfully created a Tweet'
    else
      render :new
    end
  end
end

class Topic < ActiveRecord::Base
  HOT_TOPIC_MENTIONS = 5000

  before_create :set_hot_topic, if: :hot_topic?

  protected
  def set_hot_topic
    self.hot_topic = true
  end
  def hot_topic?
    mentions > HOT_TOPIC_MENTIONS
  end
end

without_callback.rb

# Poor version without callbacks : all "hot topic" logic in the controller..
class TopicsController < ApplicationController
  def create
    @topic = Topic.new(params[:topic])  
    @topic.hot_topic = true if @topic.mentions > Topic::HOT_TOPIC_MENTIONS
    if @topic.save
      redirect_to @topic, :notice => 'Successfully created a Tweet'
    else
      render :new
    end
  end
end

class Topic < ActiveRecord::Base
  HOT_TOPIC_MENTIONS = 5000
end