CachedAt

Use cached_at for ActiveRecord cache key instead of updated at

Installation

Add this line to your application's Gemfile:

gem 'cached_at'

And then execute:

$ bundle

Or install it yourself as:

$ gem install cached_at

Too add to any class, just include CachedAt

class User
  include CachedAt
end

You will need to add the cached_at column to the table

rails generate migration add_cached_at_to_users cached_at:datetime

The cache_at column will be update when a record is created or updated.

user = User.create # sets the cached_at
user.save          # updates cached_at if another attribute on the record has changed

When a records associations are created/updated, the cached_at column will be updated but the updated_at will not change.

class Post
  belongs_to :user, touch: true
end

post = Post.create user: user # updates the user cached_at

Gem also provides class method cache_key for your model, that will generate cache key string from maximum(:cached_at)

Post.cache_key
# SELECT MAX(`posts`.`cached_at`) AS max_id FROM `posts`
# => "Post-1377768216"

This way you can pass your model class to Rails.cache.fetch as an argument to monitor when any record was changed.

Rails.cache.fetch [Post, 'last_posts'] do
  Post.last_posts
end
# => Cache fetch_hit: Post-1377768216/last_posts