134 lines
3.4 KiB
Markdown
134 lines
3.4 KiB
Markdown
---
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title: Arel
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category: Rails
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layout: 2017/sheet
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---
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### About
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{: .-intro}
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Arel is an SQL abstraction library built into Ruby on Rails.
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* <https://github.com/rails/arel>
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### Tables
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```rb
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users = Arel::Table.new(:users)
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users = User.arel_table # ActiveRecord model
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```
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### Fields
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```rb
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users[:name]
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users[:id]
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```
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### `where` (restriction)
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```rb
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users.where(users[:name].eq('amy'))
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# SELECT * FROM users WHERE users.name = 'amy'
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```
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### `select` (projection)
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```rb
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users.project(users[:id])
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# SELECT users.id FROM users
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```
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### `join`
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#### basic join
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In ActiveRecord (without Arel), if `:photos` is the name of the association, use `joins`
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```rb
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users.joins(:photos)
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```
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In Arel, if `photos` is defined as the Arel table,
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```rb
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photos = Photo.arel_table
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users.join(photos)
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users.join(photos, Arel::Nodes::OuterJoin).on(users[:id].eq(photos[:user_id]))
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```
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#### join with conditions
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```rb
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users.joins(:photos).merge(Photo.where(published: true))
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```
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If the simpler version doesn't help and you want to add more SQL statements to it:
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```rb
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users.join(
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users.join(photos, Arel::Nodes::OuterJoin)
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.on(photos[:user_id].eq(users[:id]).and(photos[:published].eq(true)))
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)
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```
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#### advanced join
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multiple `joins` with the same table but different meanings and/or conditions
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```rb
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creators = User.arel_table.alias('creators')
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updaters = User.arel_table.alias('updaters')
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photos = Photo.arel_table
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photos_with_credits = photos
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.join(photos.join(creators, Arel::Nodes::OuterJoin).on(photos[:created_by_id].eq(creators[:id])))
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.join(photos.join(updaters, Arel::Nodes::OuterJoin).on(photos[:assigned_id].eq(updaters[:id])))
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.project(photos[:name], photos[:created_at], creators[:name].as('creator'), updaters[:name].as('editor'))
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photos_with_credits.to_sql
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# => "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`)"
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# after the request is done, you can use the attributes you named
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# it's as if every Photo record you got has "creator" and "editor" fields, containing creator name and editor name
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photos_with_credits.map{|x|
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"#{photo.name} - copyright #{photo.created_at.year} #{photo.creator}, edited by #{photo.editor}"
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}.join('; ')
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```
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### `limit` / `offset`
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```rb
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users.take(5) # => SELECT * FROM users LIMIT 5
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users.skip(4) # => SELECT * FROM users OFFSET 4
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```
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### Aggregates
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```rb
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users.project(users[:age].sum) # .average .minimum .maximum
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users.project(users[:id].count)
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users.project(users[:id].count.as('user_count'))
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```
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### `order`
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```rb
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users.order(users[:name])
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users.order(users[:name], users[:age].desc)
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users.reorder(users[:age])
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```
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### With ActiveRecord
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```rb
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User.arel_table
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User.where(id: 1).arel
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```
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### Clean code with arel
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Most of the clever stuff should be in scopes, e.g. the code above could become:
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```rb
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photos_with_credits = Photo.with_creator.with_editor
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```
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You can store requests in variables then add SQL segments:
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```rb
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all_time = photos_with_credits.count
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this_month = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month))
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recent_photos = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month)).limit(5)
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```
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