class WelcomeController < ApplicationController
  def index
    @most_popular_buildings = Building.left_joins(:toilet).group(:id).order('COUNT(toilets.id) DESC').limit(5)
    @random_buildings = Building.all.order("random()").limit(5)
  end
end

class WelcomeController < ApplicationController
  def index
    @most_popular_buildings = Building.left_joins(:toilet).group(:id).order(Arel.sql("COUNT(toilets.id) DESC")).limit(5)
    @random_buildings = Building.all.order(Arel.sql("random()")).limit(5)
  end
end

Use Arel.sql on hardcoded SQL.
Use Arel.sql on hardcoded SQL.
