Let's say you visit one of your Rails app's pages in your browser and, in glancing at your logs, you happen to notice a whole bunch of these:

 (0.3ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."vehicle_id" = 1
 (0.3ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 2
 (0.2ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 3
 (1.1ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 4
 (1.0ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 5
 (1.1ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 6
 (0.5ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 7
 (0.3ms) SELECT COUNT(*) FROM "wheels" WHERE "wheels"."cohort_id" = 8

This is a sure sign that it's time to add a counter_cache. A counter cache in Rails is just an additional column that tracks the number of associated models. For instance, here's how you'd setup your two models:

class Wheel < ActiveRecord::Base
  belongs_to :vehicle, counter_cache: true

class Vehicle < ActiveRecord::Base
  has_many :wheels

Then, we need to add a migration:

class AddWheelsCountToVehicles < ActiveRecord::Migration
  def up
    add_column :vehicles, :wheels_count, :integer, default: 0, null: false

    Vehicle.find_each(select: 'id') do |result|
      Vehicle.reset_counters(result.id, :wheels)

  def down
    remove_column :vehicles, :wheels_count

This migration will add a column, named wheels_count to the vehicles table, and set the default to 0. It then proceeds to iterate through all Vehicles in the database, one-at-a-time selecting just the vehicle ID. Next, the Rails reset_counters method is called on Vehicle. Given an ID of a Vehicle and the counter_cache column to update, Rails will count the number of associated Wheel objects and update the wheels_count column.

Of course, if you have a lot of Vehicles in your database, that migration could take some time to run, so plan ahead for how you'll deploy this to your app.

blog comments powered by Disqus

Recent Posts

Subscribe for updates

Are you an iOS developer? Check out my site PushLayer, a service that makes it easy to send iOS push notifications.