新手问题有关 has_one 的一个问题

zyjisdog · 2018年04月21日 · 最后由 theblock24block 回复于 2018年04月22日 · 2504 次阅读

有两个 model 为 has_one 和 belongs_to 关系：

class User < ApplicationRecord
  has_one :second_review, dependent: :destroy
  ...
end

class SecondReview < ApplicationRecord
  belongs_to :user
  validates :user_id, presence: true, uniqueness: true  
  ...
end

现在使用user.create_second_review!来创建一条关联记录，如果user.second_review已经存在的话，数据库会报错，然后删除旧的记录，但是不会插入新记录：

2.4.0 :036 > user.create_second_review!
   (0.2ms)  BEGIN
  SecondReview Exists (0.3ms)  SELECT  1 AS one FROM `second_reviews` WHERE `second_reviews`.`user_id` = 2 LIMIT 1
   (0.1ms)  ROLLBACK
   (0.1ms)  BEGIN
  SQL (0.2ms)  DELETE FROM `second_reviews` WHERE `second_reviews`.`id` = 12
   (2.7ms)  COMMIT
ActiveRecord::RecordInvalid: 验证失败: User已经被使用
        from (irb):36

如果用 user.build_second_review.save!，同样会删除旧的记录，但是会插入新的记录：

2.4.0 :038 > user.build_second_review.save!
   (0.2ms)  BEGIN
  SQL (0.3ms)  DELETE FROM `second_reviews` WHERE `second_reviews`.`id` = 13
   (2.8ms)  COMMIT
   (0.2ms)  BEGIN
  SecondReview Exists (0.4ms)  SELECT  1 AS one FROM `second_reviews` WHERE `second_reviews`.`user_id` = 2 LIMIT 1
  SQL (0.3ms)  INSERT INTO `second_reviews` (`user_id`, `created_at`, `updated_at`) VALUES (2, '2018-04-21 08:41:17', '2018-04-21 08:41:17')
   (2.1ms)  COMMIT
 => true

如果在 model 中去掉dependent: :destroy，则不论是create还是build.save都不会有这种问题：

2.4.0 :003 > user.create_second_review!
   (0.3ms)  BEGIN
  SecondReview Exists (0.4ms)  SELECT  1 AS one FROM `second_reviews` WHERE `second_reviews`.`user_id` = 2 LIMIT 1
   (0.1ms)  ROLLBACK
   (0.1ms)  BEGIN
  SecondReview Exists (0.4ms)  SELECT  1 AS one FROM `second_reviews` WHERE `second_reviews`.`user_id` IS NULL AND (`second_reviews`.`id` != 22) LIMIT 1
   (0.3ms)  ROLLBACK
ActiveRecord::RecordNotSaved: Failed to remove the existing associated second_review. The record failed to save after its foreign key was set to nil.
        from (irb):3

2.4.0 :004 > user.build_second_review.save!
   (0.4ms)  BEGIN
  SecondReview Exists (0.4ms)  SELECT  1 AS one FROM `second_reviews` WHERE `second_reviews`.`user_id` IS NULL AND (`second_reviews`.`id` != 22) LIMIT 1
   (0.1ms)  ROLLBACK
ActiveRecord::RecordNotSaved: Failed to remove the existing associated second_review. The record failed to save after its foreign key was set to nil.
        from (irb):4

我的问题是： 1、既然 SecondReview 类里已经有 validation，为什么user_id相同的时候依然会执行数据库操作（删掉旧记录）？ 2、为什么create_second_review!方法在删除旧的记录后不会插入新记录？

对于这个问题，如果 Rails 的机制就是这样的，是不是我就只能写诸如@user.create_second_review! if [email protected]_review 这样丑陋的代码？