arel.md

   1 ---
   2 title: Arel
   3 category: Rails
   4 ---
   5
   6 ### Tables
   7
   8 ```rb
   9 users = Arel::Table.new(:users)
  10 users = User.arel_table  # ActiveRecord model
  11 ```
  12
  13 ### Fields
  14
  15 ```rb
  16 users[:name]
  17 users[:id]
  18 ```
  19
  20 ### `where` (restriction)
  21
  22 ```rb
  23 users.where(users[:name].eq('amy'))
  24 # SELECT * FROM users WHERE users.name = 'amy'
  25 ```
  26
  27 ### `select` (projection)
  28
  29 ```rb
  30 users.project(users[:id])
  31 # SELECT users.id FROM users
  32 ```
  33
  34 ### `join`
  35 #### basic join
  36 In ActiveRecord (without Arel), if `:photos` is the name of the association, use `joins`
  37 ```rb
  38 users.joins(:photos)
  39 ```
  40
  41 In Arel, if `photos` is defined as the Arel table,
  42 ```rb
  43 photos = Photo.arel_table
  44 users.join(photos)
  45 users.join(photos, Arel::Nodes::OuterJoin).on(users[:id].eq(photos[:user_id]))
  46 ```
  47
  48 #### join with conditions
  49 ```rb
  50 users.joins(:photos).merge(Photo.where(published: true))
  51 ```
  52
  53 If the simpler version doesn't help and you want to add more SQL statements to it:
  54 ```rb
  55 users.join(
  56    users.join(photos, Arel::Nodes::OuterJoin)
  57    .on(photos[:user_id].eq(users[:id]).and(photos[:published].eq(true)))
  58 )
  59 ```
  60
  61 #### advanced join
  62 multiple `joins` with the same table but different meanings and/or conditions
  63 ```rb
  64 creators = User.arel_table.alias('creators')
  65 updaters = User.arel_table.alias('updaters')
  66 photos = Photo.arel_table
  67
  68 photos_with_credits = photos
  69 .join(photos.join(creators, Arel::Nodes::OuterJoin).on(photos[:created_by_id].eq(creators[:id])))
  70 .join(photos.join(updaters, Arel::Nodes::OuterJoin).on(photos[:assigned_id].eq(updaters[:id])))
  71 .project(photos[:name], photos[:created_at], creators[:name].as('creator'), updaters[:name].as('editor'))
  72
  73 photos_with_credits.to_sql
  74 # => "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`)"
  75
  76 # after the request is done, you can use the attributes you named
  77 # it's as if every Photo record you got has "creator" and "editor" fields, containing creator name and editor name
  78 photos_with_credits.map{|x|
  79   "#{photo.name} - copyright #{photo.created_at.year} #{photo.creator}, edited by #{photo.editor}"
  80 }.join('; ')
  81 ```
  82
  83 ### `limit` / `offset`
  84
  85 ```rb
  86 users.take(5) # => SELECT * FROM users LIMIT 5
  87 users.skip(4) # => SELECT * FROM users OFFSET 4
  88 ```
  89
  90 ### Aggregates
  91
  92 ```rb
  93 users.project(users[:age].sum) # .average .minimum .maximum
  94 users.project(users[:id].count)
  95 users.project(users[:id].count.as('user_count'))
  96 ```
  97
  98 ### `order`
  99
 100 ```rb
 101 users.order(users[:name])
 102 users.order(users[:name], users[:age].desc)
 103 users.reorder(users[:age])
 104 ```
 105
 106 ### With ActiveRecord
 107
 108 ```rb
 109 User.arel_table
 110 User.where(id: 1).arel
 111 ```
 112
 113 ### Clean code with arel
 114
 115 Most of the clever stuff should be in scopes, e.g. the code above could become:
 116 ```rb
 117 photos_with_credits = Photo.with_creator.with_editor
 118 ```
 119
 120 You can store requests in variables then add SQL segments:
 121 ```rb
 122 all_time      = photos_with_credits.count
 123 this_month    = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month))
 124 recent_photos = photos_with_credits.where(photos[:created_at].gteq(Date.today.beginning_of_month)).limit(5)
 125 ```
 126
 127 ## Reference
 128
 129 * <http://github.com/rails/arel>