ruby 20 lines · 2 tabs

Counter cache for association counts

Alex Kumar Jan 2026

2 tabs

class AddCommentsCountToPosts < ActiveRecord::Migration[6.1]
  def up
    add_column :posts, :comments_count, :integer, default: 0, null: false

    # Populate existing counts
    Post.find_each do |post|
      Post.reset_counters(post.id, :comments)
    end
  end

  def down
    remove_column :posts, :comments_count
  end
end

class Comment < ApplicationRecord
  belongs_to :post, counter_cache: true
  belongs_to :author, class_name: 'User'

  validates :body, presence: true
end

2 files · ruby Explain with highlit

Computing counts on associations can be expensive when done repeatedly—post.comments.count executes a SELECT COUNT(*) query every time. Counter caches solve this by maintaining a denormalized count column on the parent model that increments/decrements automatically when children are created or destroyed. This trades a small amount of write overhead (updating the counter) for massive read performance gains. I add counter_cache: true to the belongs_to association and create a migration to add the *_count column with a default of 0. For existing data, I use reset_counters to populate initial values. Counter caches work best for read-heavy associations where counts are displayed frequently, like comment counts or like counts on posts.

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