### 测试代码：

class User < ApplicationRecord
  ...
  validate :test1
  ...
  private
    def test1
      logger.debug "=============== 测试 validate 是否被包含在事务中 ==============="
    end
end

### rails consle 调试信息：

Running via Spring preloader in process 20000
Loading development environment (Rails 5.0.0.1)
irb(main):001:0> User.create
   (0.1ms)  begin transaction
  User Exists (0.1ms)  SELECT  1 AS one FROM "users" WHERE "users"."email" IS NULL LIMIT ?  [["LIMIT", 1]]
=============== 测试 validate 是否被包含在事务中 ===============
   (0.1ms)  rollback transaction
=> #<User id: nil, name: nil, email: nil, created_at: nil, updated_at: nil, password_digest: nil, remember_digest: nil, admin: false, activation_digest: nil, activated: false, activated_at: nil, reset_digest: nil, reset_sent_at: nil>
irb(main):002:0> 

问题

回调 这个理应包含在事务中，但为什么要 验证 也要包含在事务中。在《rails tutorial》一书中，有如下一段代码：

class Micropost < ApplicationRecord
  ...
  validate :picture_size

  private
    # 验证上传的图像大小
    def picture_size
      if picture.size > 5.megabytes
        errors.add(:picture, "should be less than 5MB")
      end
    end
end

像这种 picture_size 验证，理应在事务外的。同样，实际项目中里的 model 也有许多 验证 没必要放在事务中的，那为什么 rails 却这么做了？？？