Rails 5.2升级后新实例ActiveRecord关联未自动保存问题

升级背景 #

• Rails版本：从5.0.7.2升级到5.2.8.1
• Ruby版本：从2.5.0升级到2.7.7

原工作代码（Rails 5.0中正常运行） #

def up
    Test.find_by(name: 'Rewarded_Ad_Goal').try(:destroy)
    choice_1565 = Choice.new(weight: 1, value: '0_Control')
    choice_1566 = Choice.new(weight: 1, value: '1_Alt')
    test = Test.create!(name: 'Rewarded_Ad_Goal', choices: [choice_1565, choice_1566], aasm_state: 'stopped')
end

Rails 5.2中触发的错误 #

irb(main):004:0> test = Ingot::Test.create!(name: 'Rewarded_Ad_Goal', choices: [choice_1565, choice_1566], aasm_state: 'stopped')
   (0.3ms)  BEGIN
  Ingot::Test Exists (0.4ms)  SELECT  1 AS one FROM "ingot_tests" WHERE "ingot_tests"."name" = $1 LIMIT $2  [["name", "Rewarded_Ad_Goal"], ["LIMIT", 1]]
  Ingot::Test Exists (0.4ms)  SELECT  1 AS one FROM "ingot_tests" WHERE "ingot_tests"."name" = $1 LIMIT $2  [["name", "Rewarded_Ad_Goal"], ["LIMIT", 1]]
  SQL (1.8ms)  INSERT INTO "ingot_tests" ("aasm_state", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["aasm_state", "stopped"], ["name", "Rewarded_Ad_Goal"], ["created_at", "2023-04-10 19:17:33.630832"], ["updated_at", "2023-04-10 19:17:33.630832"]]
   (0.3ms)  ROLLBACK
Traceback (most recent call last):
        1: from (irb):4
ActiveRecord::NotNullViolation (PG::NotNullViolation: ERROR:  null value in column "default_choice_id" violates not-null constraint)
DETAIL:  Failing row contains (311, null, null, stopped, null, Rewarded_Ad_Goal, 2023-04-10 19:17:33.630832, 2023-04-10 19:17:33.630832).
: INSERT INTO "ingot_tests" ("aasm_state", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"

相关模型定义 #

test.rb #

class Test < ActiveRecord::Base
    include AASM
    include Contracts

    belongs_to :default_choice, class_name: 'Ingot::Choice'
    has_many :assignments, class_name: 'Ingot::Assignment', dependent: :delete_all
    has_many :choices, class_name: 'Ingot::Choice', dependent: :destroy, autosave: true
    has_many :filters, class_name: 'Ingot::Filter', dependent: :destroy

    accepts_nested_attributes_for :default_choice,
      allow_destroy: true,
      reject_if: :all_blank

    accepts_nested_attributes_for :choices,
      allow_destroy: true,
      reject_if: :all_blank

    accepts_nested_attributes_for :filters,
      allow_destroy: true,
      reject_if: :all_blank

    validates_length_of :choices, minimum: 2, message: 'are missing.  You must specify at least two alternatives.'
    validates_presence_of :default_choice
    validates_presence_of :name
    validates_uniqueness_of :name

    before_validation :set_default_choice,
      unless: :default_choice

    def set_default_choice # confirmed this gets called
      puts "Setting default choice to #{choices.first.id}" # <== nil
      puts "Setting default choice to #{choices.first.weight}"
      puts "Setting default choice to #{choices.first.value}"
      puts "Setting default choice to #{choices.first.test_id}" # <== nil
      self.default_choice ||= choices.first
    end
end

choice.rb #

class Choice < ActiveRecord::Base
    belongs_to :test, class_name: 'Ingot::Test'
    has_many :assignments, class_name: 'Ingot::Assignment', dependent: :destroy

    validates_numericality_of :weight
    validates_presence_of :value
    validates_presence_of :weight

    before_validation :set_default_weight

    def test
      super || Test.where(default_choice_id: id).first
    end

    private

    def set_default_weight
      self.weight ||= 1
    end
end

问题分析与解决方案 #

核心原因：Rails 5.1+的关联保存顺序变更 #

在Rails 5.0中，创建父对象并关联子对象时，Rails会先保存子对象（获取id），再保存父对象，因此before_validation回调能拿到子对象的id。但从Rails 5.1开始，关联对象的保存顺序发生了关键变化：父对象会先完成验证并尝试保存，之后才处理子对象的保存。这就导致set_default_choice回调触发时，choices还只是内存中的未持久化对象，没有id和test_id，无法满足default_choice_id的非空约束。

解决办法 #

1. 调整回调逻辑，手动提前保存子对象：
   修改set_default_choice方法，先保存所有子对象再关联默认选项：

   def set_default_choice
     choices.each(&:save!) if choices.any? # 先持久化子对象
     self.default_choice ||= choices.first
   end
   

   注意：需确保子对象的验证规则都能通过，否则会抛出保存异常。

2. 改用嵌套属性创建关联对象：
   使用choices_attributes替代直接传入子对象实例，Rails会自动处理正确的保存顺序，确保default_choice能关联到已持久化的子对象：

   test = Test.create!(
     name: 'Rewarded_Ad_Goal',
     aasm_state: 'stopped',
     choices_attributes: [
       { weight: 1, value: '0_Control' },
       { weight: 1, value: '1_Alt' }
     ]
   )
   

3. 临时调整数据库约束（不推荐长期使用）：
   若允许临时修改数据库，可先将default_choice_id改为允许null，创建对象后再设置默认选项，最后改回非空约束。此方式需要额外迁移步骤，仅适合临时过渡。

内容的提问来源于stack exchange，提问作者PragmaticProgrammer