[Rails] N + 1 problems and countermeasures

What is the N + 1 problem?

For example, when trying to retrieve multiple (N) data using .each method etc., SQL is issued N times, which leads to performance degradation.

Example

Suppose you have such a one-to-many Use and Message model.

app/model/user.rb

class Message < ApplicationRecord
  has_many :messages
end

app/model/message.rb

class User < ApplicationRecord
  belongs_to :user
end

Get the value to pass to the view in the controller

app/controllers/message_controller.rb

def index
  @messages = Message.all
end

Display content and the name associated with it

app/views/index.html.erb

<% @messages.each do |m| %>
  <span><%= m.content %> </span>
  <span><%= m.user.name %> </span>
<% end %>

As for the SQL at that time, SQL will be issued for each record like this.

Message Load (3.7ms)  SELECT `messages`.* FROM `messages`　# +Part 1
  ↳ app/views/messages/index.html.erb:5
  User Load (9.4ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 1 LIMIT 1
  ↳ app/views/messages/index.html.erb:8
  User Load (1.0ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 3 LIMIT 1
  ↳ app/views/messages/index.html.erb:8
  CACHE User Load (0.0ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 3 LIMIT 1  [["id", 3], ["LIMIT", 1]]
  ↳ app/views/messages/index.html.erb:8
  CACHE User Load (0.0ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 3 LIMIT 1  [["id", 3], ["LIMIT", 1]]
  ↳ app/views/messages/index.html.erb:8
・
・
・
(Continued for N pieces)

Countermeasures

Introduce gem bullet. Then, SQL like this will be issued and performance will improve.

Message Load (1.1ms)  SELECT `messages`.* FROM `messages`
  ↳ app/views/messages/index.html.erb:5
  User Load (0.9ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` IN (1, 3, 4)