diff --git a/arel.md b/arel.md index 173833e9c..2d5b35d19 100644 --- a/arel.md +++ b/arel.md @@ -32,12 +32,54 @@ users.project(users[:id]) ``` ### `join` - +#### basic join +In ActiveRecord (without Arel), if `:photos` is the name of the association, use `joins` ```rb -users.join(photos) +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 @@ -57,6 +99,8 @@ users.project(users[:id].count.as('user_count')) ```rb users.order(users[:name]) +users.order(users[:name], users[:age].desc) +users.reorder(users[:age]) ``` ### With ActiveRecord @@ -66,6 +110,20 @@ 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) +``` + ## Reference *