Rails - Counter cache • Tuan Tran

What is counter cache?

Counter cache used to cache the number of belonging objects on associations.

How

For example, we want to count the number of comments on each post record.

class Post < ActiveRecord::Base
  has_many :comments
end

class Comments < ActiveRecord::Base
  belongs_to :post
end

The first thing you have to do is adding a comments_count column on Post table. After that, you change on Comments model:

belongs_to :post, counter_cache: true

Each time you add a new comment, this field will increase automaticly. The only problem left is that the counts are off if there are already posts and comments in the database.