fix(migration): make migration compatible with django 3 and 4 #10
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pkyosx
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可以，這個 PR 就是為了讓 3.x 跟 4.x 都相容，原先遇到的問題是不能夠在 foreign key 還在的狀態下拔掉 unique key，所以我們在前面加上一個步驟先把 foreign key 變成單純的 integer |
| for id_key in IdempotencyKey.objects.all(): | ||
| if id_key.idempotency_key in existed_keys: | ||
| id_key.delete() |
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2 questions:
- I'm wondering if it's safe to delete instances within a loop. Gemini told me it's unsafe but it doesn't provide a reference.
- I don't see any sorting options, how do you decide which one to delete when duplicated?
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- 他擔心會發生 partial failure 吧，假設後面發生錯誤但是前面已經 delete 掉一些東西了。 不過我們的邏輯相對單純不太需要擔心他跑到後面 exception 跳掉。
- 因為這個 duplication 源自於不同的 user，實際上會重複 IdempotencyKey 的狀況我們發現也都幾乎是同時發出來的，是基於 bug 導致一個 user=None 的狀況使得他認成兩個。 當我們砍掉 user 以後兩個 record 就視為同一個了，在這種狀況下不管砍掉哪一個差異不大。
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他擔心會發生 partial failure 吧
No, in most programming languages (even in Python), you shouldn't remove any item from a list during the iteration. It's because the removal changes the list itself.
I'm not sure if the django orm query support this operation.
實際上會重複 IdempotencyKey 的狀況我們發現也都幾乎是同時發出來的，是基於 bug 導致一個 user=None 的狀況使得他認成兩個
what if other columns are different? why not remove both?
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- 這個說法蠻奇怪的，因為我不是從 list 裡面移除東西，而是把 list 裡面的某個 item 內容作改變 (decoupled from DB)，Gemini 那段描述是沒有問題的，的確不應該一邊 iterate list 一邊更動 list，但是他套用的精確度有待加強。
要做到他說的事情 要走類似下面的方式 不過新版的 python 會噴就是了
> x = {i: i for i in range(3)}
> for i in x: del x[i]
Traceback (most recent call last):
File "/Users/seth_wang/.pyenv/versions/3.8.16/lib/python3.8/code.py", line 90, in runcode
exec(code, self.locals)
File "<console>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
這種手法interpreter就判斷不出來了，但是我們要操作的會是 container 本身 (list, dict, set, ...)，不是裡面的 item
> x = [1,2,3]
> for i in x: x.remove(i)
# x will be [2]
- 如果都砍掉的話我們剛好在 migrate 期間的 lock 會瞬間被清掉不受保護。 建議還是至少保留一個比較安全。
Why need this change ? / Root Cause :
When we try to migrate django 3 to django 4, the existing migration script will fail due to the following reason.
Changed Made :
Test Scope / Change Impact :