has_many 沒有指定 dependent 的時候，使用 object.associations.delete_all，其行為可能會讓你有點意外

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Pic CC-BY by Guilherme Tavares https://flic.kr/p/3AvBtV

不經意看到這篇文章 關於delete_all的Bug，把他的 git repo 抓下來試試看，行為還真的如他所述，更加引起我的好奇，因此挖進去看一下。這篇是追查的結果，也順便把我追查的過程記錄下來。

注意到其輸出的 SQL 看起來像是在進行 Nullify

DEVELOPMENT [11] rails101(main)> @group.group_users.delete_all
  SQL (0.1ms)  UPDATE "group_users" SET "group_id" = NULL WHERE "group_users"."group_id" = ?  [["group_id", 1]]
nil

找了一下 文件 卻是說 “This is a single SQL DELETE statement that goes straight to the database”，其 source code 看起來也符合其宣稱。根據文件裡提供的寫法試打看看，發現行為符合預期

DEVELOPMENT [12] rails101(main)> GroupUser.delete_all(group: @group)
  SQL (0.4ms)  DELETE FROM "group_users" WHERE "group_users"."group_id" = 1
0

看一下他們的 source code 來自何處

DEVELOPMENT [13] rails101(main)> GroupUser.method(:delete_all).source_location
[
    [0] "/usr/local/opt/rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/activerecord-4.2.5/lib/active_record/querying.rb",
    [1] 8
]

DEVELOPMENT [14] rails101(main)> @group.group_users.method(:delete_all).source_location
[
    [0] "/usr/local/opt/rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/activerecord-4.2.5/lib/active_record/associations/collection_proxy.rb",
    [1] 442
]

發現他們是來自不同檔案不同 methods:

追進 rails/activerecord/lib/active_record/associations/collection_proxy.rb 的 source code，在 method 前面的註解裡發現以下說明：

 # Deletes all the records from the collection according to the strategy
 # specified by the +:dependent+ option. If no +:dependent+ option is given,
 # then it will follow the default strategy.
 #
 # For +has_many :through+ associations, the default deletion strategy is
 # +:delete_all+.
 #
 # For +has_many+ associations, the default deletion strategy is +:nullify+.
 # This sets the foreign keys to +NULL+.

重點：

  • 如果沒給 dependent option 的話，會使用 default strategy
  • has_many 的 default strategy 是 nullify

也就是這個案例的狀況，因為原文提到 “要達成這個bug首先要把 has_many 的 dependent 參數拿掉”，且其 source code 的該 association 並未指定為 has_many :through (雖然照這幾個 tables 的開法應該是想要做 has_many_and_belongs_to 或 has_many :through？)

點進去 Blame 介面，找到以下幾個相關 commits：

  1. add CollectionProxy#delete_all documentation
  2. Do not invoke callbacks when delete_all is called
  3. Clarify deletion strategies for collection proxies

其中第 2 個 PR 裡提到這樣的用法：

DEVELOPMENT [15] rails101(main)> @group.group_users.delete_all(:delete_all)
  SQL (0.1ms)  DELETE FROM "group_users" WHERE "group_users"."group_id" = ?  [["group_id", 1]]
nil

結論

這是個 feature…