Rails 3 error warning for slow queries

notifications.rb

# config/initializers/notifications.rb
# created by @JEG2

if Rails.env == "development"
  ActiveSupport::Notifications.subscribe(
    "sql.active_record"
  ) do |_, start, finish, _, details|
    time = finish - start 
    if time >= 0.5
      raise "Slow query (#{time}s): #{details[:sql]}"
    end
  end
end