---
title: "counter_cache - count how many children a records has"
author: "Yaroslav Shmarov"
date: "2021-03-29"
canonical_url: "https://blog.superails.com/ruby-on-rails-counter_cache"
markdown_url: "https://blog.superails.com/ruby-on-rails-counter_cache.md"
description: "conter_cache in Ruby on Rails the easy way"
tags: ["ruby","rails","ruby-on-rails"]
license: "GNU AGPL v3. Credit Yaroslav Shmarov and link the canonical URL."
video_url: "https://www.youtube.com/watch?v=30FZFU3oTiQ"
---

# counter_cache - count how many children a records has

Often you want to count how many child records a record has (`user.posts.count`, `user.comments.count`). 

Storing this data in a the database is more efficient (like `user.posts_count`, `user.comments_count`) than recalculating it each time. 

`counter_cache` gives us a way to recalculate the database field containing count of child records whenever a child record is created/deleted

### HOWTO

user.rb
```
has_many :posts
```
post.rb - add `counter_cache: true` to recalculate `posts_count` field in user table
```
belongs_to :user, counter_cache: true
```
console:
```
rails g migration add_posts_count_to_users posts_count:integer
```
migration:
```
add_column :users, :posts_count, :integer, default: 0, null: false
```
rails c - recalculate posts_count for all existing `posts` and `users`
```
User.find_each { |u| User.reset_counters(u.id, :posts) }
```