Proper Counter Cache Migrations in Rails Feb 5, 2016

This post is less about the basics of counter_cache support in Rails and more about best practices for introducing a new counter cache to an existing project. More specifically, the goal is to detail the most efficient way way to create an ActiveRecord migration to support a new counter cache column.

The Problem

You see, the interweb is currently filled with some poor advice about how to seed a counter cache column.

Now that you’ve clicked on those examples, please erase them from your memory. Iterating over the entire table, loading each record into ruby-space (without batching, mind you), and relying on either update_counters or reset_counters in your migration is a sure way for your next deployment to take minutes to finish. It doesn’t take millions of records to get hit with this pain either.

The Example

Let’s assume that we have some stereotypical tables named posts and comments. Let’s also assume that we decided to add a comments_count counter cache column to the posts table.

The Migration

Given this example, your migration should look something like this:

class AddCommentsCountToPosts < ActiveRecord::Migration
  def change
    change_table :posts do |t|
      t.integer :comments_count, default: 0

    reversible do |dir|
      dir.up { data }

  def data
    execute <<-SQL.squish
        UPDATE posts
           SET comments_count = (SELECT count(1)
                                   FROM comments
                                  WHERE comments.post_id =

The Results

With over to 25,000 posts and 100,000 comments, using SQL will take seconds instead of minutes.

-- execute("UPDATE posts SET comments_count = (SELECT count(1) FROM comments WHERE comments.post_id =")
   -> 1.3197s
   -> 26900 rows

Let’s compare that with how long it would have taken if we used a Post.reset_counters approach:

-- Seeding posts.comments_count -- Better grab a coffee.
   -> ...........................
   -> 144.7302s
   -> 26900 rows

Here is the actual code used to run these two examples.

The Moral

Sometimes SQL can be straightforward and fun.

Please share, reply to comment, or retweet. Also follow me for more.