我正在使用下面列出的Rails代码获得非常难看的SQL查询：
Facility.includes(:type, :owner_building, :delegated_building, keeper_building, :owner_user, :keeper_user).order('users.name ASC').all
它产生：
SELECT `facilities`.`id` AS t0_r0, `facilities`.`name` AS t0_r1, `facilities`.`brand` AS t0_r2, `facilities`.`desc` AS t0_r3, `facilities`.`type_id` AS t0_r4, `facilities`.`owner_building_id` AS t0_r5, `facilities`.`keeper_building_id` AS t0_r6, `facilities`.`delegated_building_id` AS t0_r7, `facilities`.`owner_user_id` AS t0_r8, `facilities`.`keeper_user_id` AS t0_r9, `buildings`.`id` AS t1_r0, `buildings`.`name` AS t1_r1, `buildings`.`address` AS t1_r2, `buildings`.`created_at` AS t1_r3, `buildings`.`updated_at` AS t1_r4, `buildings`.`comments` AS t1_r5, `delegated_buildings_facilities`.`id` AS t2_r0, `delegated_buildings_facilities`.`name` AS t2_r1, `delegated_buildings_facilities`.`address` AS t2_r2, `delegated_buildings_facilities`.`created_at` AS t2_r3, `delegated_buildings_facilities`.`updated_at` AS t2_r4, `delegated_buildings_facilities`.`comments` AS t2_r5, `keeper_buildings_facilities`.`id` AS t3_r0, `keeper_buildings_facilities`.`name` AS t3_r1, `keeper_buildings_facilities`.`address` AS t3_r2, `keeper_buildings_facilities`.`created_at` AS t3_r3, `keeper_buildings_facilities`.`updated_at` AS t3_r4, `keeper_buildings_facilities`.`comments` AS t3_r5, `users`.`id` AS t4_r0, `users`.`company_id` AS t4_r1, `users`.`building_id` AS t4_r2, `users`.`login` AS t4_r3, `users`.`name` AS t4_r4, `users`.`role` AS t4_r5, `users`.`email` AS t4_r6, `users`.`comments` AS t4_r7, `users`.`crypted_password` AS t4_r8, `users`.`password_salt` AS t4_r9, `users`.`persistence_token` AS t4_r10, `users`.`perishable_token` AS t4_r11, `users`.`login_count` AS t4_r12, `users`.`failed_login_count` AS t4_r13, `users`.`last_request_at` AS t4_r14, `users`.`current_login_at` AS t4_r15, `users`.`last_login_at` AS t4_r16, `users`.`current_login_ip` AS t4_r17, `users`.`last_login_ip` AS t4_r18, `users`.`created_at` AS t4_r19, `users`.`updated_at` AS t4_r20, `keeper_users_facilities`.`id` AS t5_r0, `keeper_users_facilities`.`company_id` AS t5_r1, `keeper_users_facilities`.`building_id` AS t5_r2, `keeper_users_facilities`.`login` AS t5_r3, `keeper_users_facilities`.`name` AS t5_r4, `keeper_users_facilities`.`role` AS t5_r5, `keeper_users_facilities`.`email` AS t5_r6, `keeper_users_facilities`.`comments` AS t5_r7, `keeper_users_facilities`.`crypted_password` AS t5_r8, `keeper_users_facilities`.`password_salt` AS t5_r9, `keeper_users_facilities`.`persistence_token` AS t5_r10, `keeper_users_facilities`.`perishable_token` AS t5_r11, `keeper_users_facilities`.`login_count` AS t5_r12, `keeper_users_facilities`.`failed_login_count` AS t5_r13, `keeper_users_facilities`.`last_request_at` AS t5_r14, `keeper_users_facilities`.`current_login_at` AS t5_r15, `keeper_users_facilities`.`last_login_at` AS t5_r16, `keeper_users_facilities`.`current_login_ip` AS t5_r17, `keeper_users_facilities`.`last_login_ip` AS t5_r18, `keeper_users_facilities`.`created_at` AS t5_r19, `keeper_users_facilities`.`updated_at` AS t5_r20, `facility_types`.`id` AS t6_r0, `facility_types`.`name` AS t6_r1, `facility_types`.`desc` AS t6_r2, `facility_migrations`.`id` AS t7_r0, `facility_migrations`.`building_id` AS t7_r1, `facility_migrations`.`equipment_id` AS t7_r2, `facility_migrations`.`facility_id` AS t7_r3, `facility_migrations`.`created_at` AS t7_r4
FROM `facilities`
LEFT OUTER JOIN`buildings` ON `buildings`.`id` = `facilities`.`owner_building_id`
LEFT OUTER JOIN`buildings` `delegated_buildings_facilities` ON `delegated_buildings_facilities`.`id` = `facilities`.`delegated_building_id`
LEFT OUTER JOIN`buildings` `keeper_buildings_facilities` ON `keeper_buildings_facilities`.`id` = `facilities`.`keeper_building_id`
LEFT OUTER JOIN`users` ON `users`.`id` = `facilities`.`owner_user_id`
LEFT OUTER JOIN`users` `keeper_users_facilities` ON `keeper_users_facilities`.`id` = `facilities`.`keeper_user_id`
LEFT OUTER JOIN`facility_types` ON `facility_types`.`id` = `facilities`.`type_id`
LEFT OUTER JOIN`facility_migrations` ON `facility_migrations`.`facility_id` = `facilities`.`id`
WHERE `facilities`.`id` IN (15, 47, 16, 48, 17, 49, 18, 50, 19, 51, 20, 52) AND ((1=1)) ORDER BY users.name ASC
那么我怎样才能将LEFT JOIN仅用于我有条件的字段（如排序）和其他表的简单SELECT（如果没有条件则定期工作）？
答案 0 :(得分：2)
我不完全理解这个问题;但你的意思是丑陋的“选择A为B，X为Y”？
如果是这样，那么这是将返回值正确映射到其对应对象的Rails（AR）方式。任何t0_r *都将映射到一个设施对象，任何t1_r *都将映射到一个建筑物对象等......
使用select无法帮助您。
Rails内部使用此查询，据我所知，其他ORM也是如此（例如Hibernate）。
答案 1 :(得分：2)
不幸的是，您无法将联接（：关联）（或选择，分组等）与急切的加载工具“includes（：associations）”组合在一起。因此，一种解决方案是在2中减少查询，这总是比延迟加载的O（n）效果更好或加载大量不需要的activerecord对象（非常昂贵）：
第一次获取过滤后的设施ID，而不需要加载
facilities_ids = Facility.select("facilities.id").
joins([:conditional_associations, ...]).map(&:id)
然后使用它们来过滤您热切的查询，其中包含“包含”的所有优点：
@facilities = Facility.includes([:loaded_associations]).
where(["facilities.id IN (?)", facilities_ids])
答案 2 :(得分：0)
这是一个数据模型问题，而不是AR问题。 AR只是暴露了可能的架构设计问题。
就个人而言，我会调查为什么deligated_build与建筑物不同。它们应该在同一个表中，然后您可以在查询子句中指定哪个委派的构建ID而不是连接。类似的_building表也是如此。
答案 3 :(得分：0)
似乎没人提到它。有一种名为preload的方法与includes几乎相同，只是它使用单独的查询而不是左连接。
如果您希望左外连接仅用于排序/分组/过滤而不包含在结果中，您可能希望使用squeel gem，它支持joins方法中的外连接。