Sometimes, for better performance, you may need database redundancy, which means you will store the same information in different tables. For example, you may want to save username into posts table, rather than just store user_id in it.

Redundancy allows you to quickly make a cache column in ActiveRecord.

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name

      t.timestamps
    end
  end
end

class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.references :user, index: true
      t.string :user_name
      
      t.string :title
      t.text :content

      t.timestamps
    end
  end
end

class User < ActiveRecord::Base
  has_many :posts

  def raw_posts_count
    posts.count
  end
end

class Post < ActiveRecord::Base
  belongs_to :user

  cache_column :user, :name
  cache_method :user, :posts_count
end

Here Redundancy do 2 things for you:

1. when we create or update a post, it will check post.user_id_changed?, if true, it will update user_name by calling post.write_attribute(:user_name, post.user.name)
2. when we update a user, it will check user.name_changed?, if true, it will update the user_name of all related posts by calling user.posts.update_all(:user_name => user.name)

cache_column association, attribute, options

available options:

1. cache_column - Specify the column used to store the cached attribute, by default this is :"#{association}_#{attribute}", e.g.: user.name will cached in post.user_name

cache_method association, attribute, options

available options:

1. cache_method - Specify the method need to be cached, by default this is :"raw_#{attribute}", e.g.: user.raw_orders_count will cached in post.orders_count

Install the gem by adding it to your Gemfile and bundle it up:

gem 'activerecord-redundancy', github: 'bbtfr/activerecord-redundancy'

And you are ready to go.

Note: This gem is only tested on Rails 3.2 and 4.

This project rocks and uses MIT-LICENSE.