ruby-on-rails - rails : why cannot delete dependent table if dependent: :destroy allowed?

Question

article_id 必须在 Pictures 表中，因为每篇文章可以有许多图片。

所以图片表看起来像这样:

id        article_id         picture

1           34              pinguin.jpg
2           56              koala.jpg
3           56              bear.jpg

等等

我知道destroy_all(它在依赖表上实例化一个对象并调用它的destroy方法，这是一个缓慢的过程)和delete_all之间的区别，它只是清除记录而不关心关联。

但我必须使用 destroy_all 因为有关联的依赖记录:

Article Model
    class Article < ActiveRecord::Base
      belongs_to :user
      has_many :pictures , **`dependent: :destroy`**
      accepts_nested_attributes_for :pictures

Picture Model
    class Picture < ActiveRecord::Base
      belongs_to :article
     mount_uploader :picture, PictureUploader

     def destroy
     end

架构

 create_table "articles", force: :cascade do |t|
    t.string   "country"
    t.string   "region"
    t.string   "town"
    t.string   "street"
    t.string   "company"
    t.string   "title"
    t.text     "content"
    t.integer  "user_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  add_index "articles", ["user_id", "created_at"], name: "index_articles_on_user_id_and_created_at", using: :btree
  add_index "articles", ["user_id"], name: "index_articles_on_user_id", using: :btree

  create_table "pictures", force: :cascade do |t|
    t.integer  "article_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.string   "picture"
  end

所以，这两个命令，Article.delete_all 和 Article.destroy_all

提出类似的投诉

Article.destroy_all
  Article Load (0.9ms)  SELECT "articles".* FROM "articles"  ORDER BY "articles"."created_at" DESC
   (0.1ms)  BEGIN
  SQL (1.7ms)  DELETE FROM "articles" WHERE "articles"."id" = $1  [["id", 21]]
PG::ForeignKeyViolation: ERROR:  update or delete on table "articles" violates foreign key constraint "fk_rails_658164416e" on table "pictures"
DETAIL:  Key (id)=(21) is still referenced from table "pictures".
: DELETE FROM "articles" WHERE "articles"."id" = $1
   (0.2ms)  ROLLBACK


class CreatePictures < ActiveRecord::Migration
  def change
    create_table :pictures do |t|
      t.references :article, index: true, foreign_key: true

      t.timestamps null: false
    end
  end
end

更新

我曾尝试在迁移文件中添加 on delete 级联 FK，但无济于事:

我首先删除了 FK，因为我必须添加限定符:on_delete::cascade

class RemoveFkFromPictures < ActiveRecord::Migration
  def change

    remove_foreign_key :pictures, column: :article_id
  end
end

我创建了另一个迁移文件来添加 FK

class AddForeignKeyToPictures1 < ActiveRecord::Migration
  def change
    add_foreign_key :pictures, :articles, on_delete: :cascade , name: :deletingarticlescascadesonpictures

  end
end

模式看起来像:

add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree

  add_foreign_key "articles", "users"
  add_foreign_key "pictures", "articles", name: "deletingarticlescascadesonpictures", on_delete: :cascade
end

所以，你会认为现在可以了。好吧，它不是:

  Article Load (1.3ms)  SELECT "articles".* FROM "articles"  ORDER BY "articles"."created_at" DESC
   (0.2ms)  BEGIN
  Picture Load (0.5ms)  SELECT "pictures".* FROM "pictures" WHERE "pictures"."article_id" = $1  [["article_id", 25]]
   (0.4ms)  ROLLBACK
ActiveRecord::RecordNotDestroyed: Failed to destroy the record

Answer 1

首先，我要感谢您的洞察力和乐于助人的良好意愿。

从你离开的地方开始，我告诉你我是如何解决这个问题的，我花了两天时间，直到今天醒来，我想我可以尝试一下。

1) 事实上，在 Parent 模型上只写依赖::destroy 是不够的。当您实际上是指“删除，级联”时，Ruby 似乎实际上并没有完成“不留下孤儿”的工作，被认为是用“销毁”来表示的。

2)当您生成创建 Fk 的模型时，例如 user:references，它确实创建了一个 FK，您可以在 schema.rb 文件中看到它，但是，正如您所指出的，默认情况下是“限制”，即执行对 child 没什么。

3)然后我们就完蛋了。因为Herodes 和FK 都不会破坏。

4)您需要从迁移文件中删除 FK，将其替换为带有“on delete cascade”的 FK

和

5)您需要删除仍然位于模型中的依赖::销毁。

6) 这样，低级数据库命令将接管并且删除进行干净和快速。