Sometimes calculating the count of a collection on every page render, even in SQL, is not the best option.
Imagine that you need to render a post, with its count of likes and comments, but the post has thousands of likes and comments. That would be slow.
One solution is to save the count of those comments and likes in the record, and when you need to render the post, instead of counting the elements again, just display the count that you already saved.
Keeping that count in sync is not easy, but rails makes it easy for us with a feature called “counter cache”.
Here I will show you how you can use a counter caches, how to introduce it in an existent association and some tips to work with them.
To add a counter cache you will need to:
counter_cache: true option in the belongs_to declaration.has_many), with the name #{table_name}_count.For example to keep the count of likes in a post, you can add a likes_count coulumn in the posts table.
class Post < ApplicationRecord
has_many :likes
end
class Like < ApplicationRecord
# 1. Add the counter_cache: true option in the belongs_to declaration
belongs_to :post, counter_cache: true
end
class CreatePosts < ActiveRecord::Migration[7.0]
def change
create_table :posts do |t|
#...
# 2. Add a column to keep the count in the associated model (has_many)
t.integer :likes_count
end
create_table :likes do |t|
t.references :post
end
end
end
If you want to add a different name to the counter cache column you have to specify the name of the column in both association declarations, the belongs_to and the has_many.
class Post < ApplicationRecord
# Specify the custom column name in the has_many association
has_many :likes, counter_cache: :likes_total
end
class Like < ApplicationRecord
# Specify the custom column name in the belongs_to association
belongs_to :post, counter_cache: :likes_total
end
class CreatePosts < ActiveRecord::Migration[7.0]
def change
create_table :posts do |t|
#...
# 2. Add a column to keep the count in the associated model (has_many)
t.integer :likes_total
end
create_table :likes do |t|
t.references :post
end
end
end
It is important to remember to also define the counter_cache option in the has_many association. Without it, the size method won’t be able to read your counter cache column.
If you want to add a counter cache to an existing association that already has records created, you will need to populate the counter column.
If you don’t have too many records to update you can use the reset_counters class method.
You can iterate through each record and reset its counter.
class Post < ApplicationRecord
has_many :likes
end
class Like < ApplicationRecord
# 1. Add the counter_cache: true option in the belongs_to declaration
belongs_to :post, counter_cache: true
end
class AddLikesCountToPosts < ActiveRecord::Migration[7.0]
def up
# 2. Add a column to keep the count in the associated model (has_many)
add_column :posts, :likes_count, :integer
# 3. Update the counters for each post
Post.find_each do |post|
Post.reset_counters(post.id, :likes, touch: true) # The touch is optional
end
end
def down
remove_column :posts, :likes_count, :integer
end
end
The reset_counters method will make a database call for each post, and if you have many records it will take a lot of time. Remember to just use this method if you don’t have too many records.
As I said before, If you will add the counter cache to an existing association that already has records, you will need to populate the counter column.
But If you have too many records to update, the previous method can be really slow, because it will make a database call for each record.
Other way of accomplish the task is by executing sql directly, like it is described by Ryan McGeary in the post Proper Counter Cache Migrations in Rails.
class Post < ApplicationRecord
has_many :likes
end
class Like < ApplicationRecord
# 1. Add the counter_cache: true option in the belongs_to declaration
belongs_to :post, counter_cache: true
end
class AddLikesCountToPosts < ActiveRecord::Migration[7.0]
def up
# 2. Add a column to keep the count in the associated model (has_many)
add_column :posts, :likes_count, :integer
# 3. Update the counters for each post
update_counters
end
def down
remove_column :posts, :likes_count, :integer
end
def update_counters
execute <<-SQL.squish
UPDATE posts
SET likes_count = (
SELECT count(1)
FROM likes
WHERE likes.post_id = posts.id
)
SQL
end
end
You can use it in two ways:
size methodclass Post < ApplicationRecord
has_many :likes
end
class Like < ApplicationRecord
belongs_to :post, counter_cache: true
end
posts = Post.limit(10)
posts.each do |post|
# Using the method with the column name
post.likes_count
# "size" will also read you counter cache column
post.likes.size
end
If you are using a custom column name, you can also use both, the column name and the size method.
class Post < ApplicationRecord
has_many :likes, counter_cache: :total_likes
end
class Like < ApplicationRecord
belongs_to :post, counter_cache: :total_likes
end
posts = Post.limit(10)
posts.each do |post|
# Using the method with the column name
post.total_likes
# "size" will also read you counter cache column
post.likes.size
end
But, when you are using a custom counter cache name, is important to remember to also define the counter_cache option in the has_many association. Without it size won’t be able to read your counter cache column.
class Post < ApplicationRecord
# If you don't define the custom counter cache column...
has_many :likes
end
class Like < ApplicationRecord
belongs_to :post, counter_cache: :total_likes
end
posts = Post.limit(10)
posts.each do |post|
# "size" won't know the custom column name and will execute a COUNT query
post.likes.size
end
Learn just enough fundamentals to be fluent preloading associations with ActiveRecord, and start helping your team to avoid n+1 queries on production.