> For the complete documentation index, see [llms.txt](https://huang-jason.gitbook.io/ruby-rails-syntax/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://huang-jason.gitbook.io/ruby-rails-syntax/todo-n+1.md).

# TODO N+1

<https://code.tutsplus.com/articles/improving-the-performance-of-your-rails-app-with-eager-loading--cms-25018>

## Introduction

Ruby on Rails ORM has lazy loading enabled.

## Example

```ruby
#  Post Model
class Post < ActiveRecord::Base
    belongs_to :author
end

#  Author Model
class Author < ActiveRecord::Base
    has_many :posts
end
```

```ruby
# Controller
class PostsController < ApplicationController
    def index
        @posts = Post.order(created_at: :desc)
    end
end
```

```ruby
# View
<% @posts.each do |post| %>
    <td><%= post.title %></td>
    <td><%= post.author.name %></td>
<% end %>
```

```
authors = Author.create([{ name: 'John' }, { name: 'Doe' }, { name: 'Manish' }])
Post.create(title: 'I love Tuts+', body: '', author: authors.first)
Post.create(title: 'Tuts+ is Awesome', body: '', author: authors.second)
Post.create(title: 'Long Live Tuts+', body: '', author: authors.last)
```

## Case 1: Default - lazy loading

```ruby
@posts = Post.order(created_at: :desc)
```

N+1

```ruby
Post Load (0.6ms)  SELECT "posts".* FROM "posts"  ORDER BY "posts"."created_at" DESC
Author Load (0.5ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 3]]
Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 2]]
Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 1]]
```

## Case 2.1: `preload` - eager loading / separated queries

```ruby
@posts = Post.order(created_at: :desc).preload(:author)
```

```ruby
SELECT "posts".* FROM "posts"  ORDER BY "posts"."created_at" DESC
SELECT "authors".* FROM "authors" WHERE "authors"."id" IN (3, 2, 1)
```

1. No N+1
2. Fail @ order posts by author name
3. Fail @ find posts by author name

## Case 2.2: `eager_load` - eager loading / one query

```ruby
@posts = Post.order(created_at: :desc).eager_load(:author)
```

```ruby
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."author_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "authors"."id" AS t1_r0, "authors"."name" AS t1_r1, "authors"."created_at" AS t1_r2, "authors"."updated_at" AS t1_r3 
FROM "posts"
LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id" 
ORDER BY "posts"."created_at" DESC
```

order posts by author name

```ruby
@posts = Post.order("authors.name").eager_load(:author)
```

```ruby
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."author_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "authors"."id" AS t1_r0, "authors"."name" AS t1_r1, "authors"."created_at" AS t1_r2, "authors"."updated_at" AS t1_r3 
FROM "posts"
LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
ORDER BY authors.name
```

find posts by author name

```ruby
@posts = Post.order(created_at: :desc).eager_load(:author).where("authors.name = ?", "Manish")
```

```ruby
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."author_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "authors"."id" AS t1_r0, "authors"."name" AS t1_r1, "authors"."created_at" AS t1_r2, "authors"."updated_at" AS t1_r3 
FROM "posts"
LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
WHERE (authors.name = 'Manish')  
ORDER BY "posts"."created_at" DESC
```

**single query and** `LEFT OUTER JOIN`**can also be very expensive.**

## Case 2.3 `includes` **- smarter**

(same as preload)

```ruby
@posts = Post.order(created_at: :desc).includes(:author)
```

```ruby
SELECT "posts".* FROM "posts"  ORDER BY "posts"."created_at" DESC
SELECT "authors".* FROM "authors" WHERE "authors"."id" IN (3, 2, 1)
```

order posts by author name (same as eager\_load)

```ruby
@posts = Post.order("authors.name").includes(:author)
```

```ruby
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."author_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "authors"."id" AS t1_r0, "authors"."name" AS t1_r1, "authors"."created_at" AS t1_r2, "authors"."updated_at" AS t1_r3 
FROM "posts"
LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
ORDER BY authors.name
```

find posts by author name (same as eager\_load)

```ruby
@posts = Post.order(created_at: :desc).includes(:author).where("authors.name = ?", "Manish").references(:author)
```

```ruby
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."author_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "authors"."id" AS t1_r0, "authors"."name" AS t1_r1, "authors"."created_at" AS t1_r2, "authors"."updated_at" AS t1_r3 
FROM "posts"
LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
WHERE (authors.name = 'Manish')  
ORDER BY "posts"."created_at" DESC
```