#36168: Backwards migration to replaced migration when other app has squashed
migrations can lead to FieldDoesNotExist error due to incorrect state
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Reporter: Klaas van Schelven | Owner: (none)
Type: Bug | Status: new
Component: Migrations | Version: 5.1
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Jacob Walls):
Hi Klaas. Sorry if my shorthand was not helpful. What I meant was that I
tested your suggested patch against your test project and found that I
still couldn't migrate backward to 0001_initial, finding instead this
exception:
{{{#!py
File "/Users/.../django/django/db/migrations/executor.py", line 229, in
_migrate_all_backwards
self.unapply_migration(states[migration], migration, fake=fake)
~~~~~~^^^^^^^^^^^
KeyError: <Migration triggerfailingcode.0002_baz_baz>
}}}
Because your patch (and the changes in #24900) take place inside a
[https://github.com/django/django/blob/14b46c1b848d846fad55c8634daeabd8787cf0d6/django/db/migrations/executor.py#L31
loop over migration targets], by "pre-walking" I wondered if we could
avoid rebuilding the graph during the loop and instead just derive
whatever information we need to build the graph correctly only once, but I
wasn't saying I had confidence in this hypothesis, I was just asking if
you had already explored it. From the PR review comments in #24900 it
appears the original version was quite different and perhaps more like
what I'm suggesting now. (It's hard to say since I don't have the branch
anymore.)
Thanks for looking into it, I'd be glad to review a PR.
--
Ticket URL: <https://code.djangoproject.com/ticket/36168#comment:9>
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