---
title: Arel
category: Rails
layout: 2017/sheet
---

### About
{: .-intro}

Arel is an SQL abstraction library built into Ruby on Rails.

* <https://github.com/rails/arel>

### Tables

```rb
users = Arel::Table.new(:users)
users = User.arel_table  # ActiveRecord model
```

### Fields

```rb
users[:name]
users[:id]
```

### `where` (restriction)

```rb
users.where(users[:name].eq('amy'))
# SELECT * FROM users WHERE users.name = 'amy'
```

### `select` (projection)

```rb
users.project(users[:id])
# SELECT users.id FROM users
```

### `join`
#### basic join
In ActiveRecord (without Arel), if `:photos` is the name of the association, use `joins`
```rb
users.joins(:photos)
```

In Arel, if `photos` is defined as the Arel table,
```rb
photos = Photo.arel_table
users.join(photos) 
users.join(photos, Arel::Nodes::OuterJoin).on(users[:id].eq(photos[:user_id]))
```

#### join with conditions
```rb
users.joins(:photos).merge(Photo.where(published: true))
```

If the simpler version doesn't help and you want to add more SQL statements to it:
```rb
users.join(
   users.join(photos, Arel::Nodes::OuterJoin)
   .on(photos[:user_id].eq(users[:id]).and(photos[:published].eq(true)))
)
```

#### advanced join
multiple `joins` with the same table but different meanings and/or conditions
```rb
creators = User.arel_table.alias('creators')
updaters = User.arel_table.alias('updaters')
photos = Photo.arel_table

photos_with_credits = photos
.join(photos.join(creators, Arel::Nodes::OuterJoin).on(photos[:created_by_id].eq(creators[:id])))
.join(photos.join(updaters, Arel::Nodes::OuterJoin).on(photos[:assigned_id].eq(updaters[:id])))
.project(photos[:name], photos[:created_at], creators[:name].as('creator'), updaters[:name].as('editor'))

photos_with_credits.to_sql
# => "SELECT `photos`.`name`, `photos`.`created_at`, `creators`.`name` AS creator, `updaters`.`name` AS editor FROM `photos` INNER JOIN (SELECT FROM `photos` LEFT OUTER JOIN `users` `creators` ON `photos`.`created_by_id` = `creators`.`id`) INNER JOIN (SELECT FROM `photos` LEFT OUTER JOIN `users` `updaters` ON `photos`.`updated_by_id` = `updaters`.`id`)"

# after the request is done, you can use the attributes you named
# it's as if every Photo record you got has "creator" and "editor" fields, containing creator name and editor name
photos_with_credits.map{|x|
  "#{photo.name} - copyright #{photo.created_at.year} #{photo.creator}, edited by #{photo.editor}"
}.join('; ')
```

### `limit` / `offset`

```rb
users.take(5) # => SELECT * FROM users LIMIT 5
users.skip(4) # => SELECT * FROM users OFFSET 4
```

### Aggregates

```rb
users.project(users[:age].sum) # .average .minimum .maximum
users.project(users[:id].count)
users.project(users[:id].count.as('user_count'))
```

### `order`

```rb
users.order(users[:name])
users.order(users[:name], users[:age].desc)
users.reorder(users[:age])
```

### With ActiveRecord

```rb
User.arel_table
User.where(id: 1).arel
```

### Clean code with arel

Most of the clever stuff should be in scopes, e.g. the code above could become:
```rb
photos_with_credits = Photo.with_creator.with_editor
```

You can store requests in variables then add SQL segments:
```rb
all_time      = photos_with_credits.count
this_month    = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month))
recent_photos = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month)).limit(5)
```