ruby-on-rails - PG::ForeignKeyViolation: 错误:更新或删除表 "xxx"违反了外键约束

Question

我有几个表具有与之关联的外键约束，每个表都以分层方式引用另一个表，如下所述。

当我试图销毁一家至少有 1 个项目、至少有 1 个任务、至少有 1 个任务时间的公司时...

irb(main):014:0> Company.first.destroy

我得到以下输出和错误。我现在的印象是简单地拥有 dependent: :delete_all 不处理外键约束，这是真的吗？如果是这样，我该如何处理这种情况？我知道 before_destroy 回调，在这种情况下我必须使用它吗？如果是这样，我如何暂时禁用外键约束以破坏所有关联的行？更令人困惑的是，我有一个旧的 Rails 项目，它设置了相同的表/模型，只是它是一个 model_a has_many model_bs, dependent: delete_all 与外键约束的关系，我可以ModelB.destroy_all 它有效，所以我不明白。我还读过在表上设置级联删除的帖子，还有一些帖子说如果你自己在代码中处理它就不需要这样做；如果解决方案不太复杂，我想在我的代码中处理这个问题。

Company Load (0.4ms)  SELECT  "companies".* FROM "companies" ORDER BY 
                              "companies"."id" ASC LIMIT $1 [["LIMIT", 1]]
   (0.2ms)  BEGIN
             SQL (0.9ms)  DELETE FROM "projects" 
                          WHERE "projects"."company_id" = $1 [["company_id", 3]]
   (0.1ms)  ROLLBACK
             Traceback (most recent call last):
   1: from (irb):13
             ActiveRecord::InvalidForeignKey (PG::ForeignKeyViolation: ERROR:  update or delete on table "projects" violates foreign key constraint "fk_rails_02e851e3b7" on table "tasks"
                          DETAIL:  Key (id)=(4) is still referenced from table "tasks".
                        : DELETE FROM "projects" WHERE "projects"."company_id" = $1)

架构

# /db/schema.rb

create_table "companies", force: :cascade do |t|
...
end

create_table "projects", force: :cascade do |t|
...
end

create_table "tasks", force: :cascade do |t|
...
end

create_table "task_times", force: :cascade do |t|
...
end
...

add_foreign_key "projects", "companies"
add_foreign_key "tasks", "projects"
add_foreign_key "task_times", "tasks"

模型

# /models/company.rb

class Company < ApplicationRecord
  has_many :projects, dependent: :delete_all
...
end

# /models/project.rb

class Project < ApplicationRecord
  has_many :tasks, dependent: :delete_all
...
end

# /models/task.rb

class Task < ApplicationRecord
  has_many :task_times, dependent: :delete_all
...
end

# /models/task_time.rb

class TaskTime < ApplicationRecord
...
end

Answer 1

来自fine manual :

has_many(name, scope = nil, options = {}, &extension)
[...]

• :dependent
  Controls what happens to the associated objects when their owner is destroyed. Note that these are implemented as callbacks, and Rails executes callbacks in order. Therefore, other similar callbacks may affect the :dependent behavior, and the :dependent behavior may affect other callbacks.
  • :destroy causes all the associated objects to also be destroyed.
  • :delete_all causes all the associated objects to be deleted directly from the database (so callbacks will not be executed).
  • [...]

所以 :delete_all 确实处理了外键，但是，由于没有调用回调，它只深入一层。所以在 Company 中:

has_many :projects, dependent: :delete_all

表示对公司调用#destroy 将直接从数据库中删除关联的项目。但是那不会看到这个:

has_many :tasks, dependent: :delete_all

您在 Project 中拥有的内容，并且您最终尝试删除仍在 tasks 中引用的项目，如错误消息所示。

您可以将所有关联切换为dependent::destroy，这将在销毁它们之前将所有内容从数据库中拉出并调用回调(这将从数据库中加载更多内容只是为了销毁它们，这将从数据库中加载更多的东西......)。最终结果将是大量的数据库事件，但所有外键都将得到正确遵循。

或者，您可以通过指定 on delete cascade on the foreign key constraints 将逻辑放在它通常所属的数据库中。 :

CASCADE specifies that when a referenced row is deleted, row(s) referencing it should be automatically deleted as well

您的 add_foreign_key 调用如下所示:

add_foreign_key "projects", "companies", on_delete: :cascade
add_foreign_key "tasks", "projects", on_delete: :cascade
add_foreign_key "task_times", "tasks", on_delete: :cascade

在这种情况下。您可能希望在模型中保留 dependent: :delete_all 作为正在发生的事情的提醒，或者您可以给自己留下评论。