#23474: Schema migrations can inadvertently destroy data
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Reporter: Naddiseo | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7
Severity: Release blocker | 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 carljm):
I think akaariai's idea of requiring more explicit intent to rollback
migrations is worth consideration, but entirely apart from that it still
seems to me that the previous behaviour was bad and this PR made it
better.
In #23410 I see that Andrew stated that the previous behavior was
intentional, to wit: "the contract is that backwards migrations roll back
to just after the application of the named migration, and all its
dependencies are unapplied."
I don't see any clear reasoning provided there as to _why_ that contract
is more desirable than the alternatives. Based on the following paragraph,
it _seems_ that the reasoning is that a "graph node specification" such as
"appname 0001" should always precisely specify a single state, regardless
of whether you are rolling backwards or forwards to that state.
I don't find that convincing. Although I see the purity argument, I think
that in practice a much more useful (and intuitive, and less likely to
result in data loss) contract is that "migrate appname 0001" will ensure
that appname-0001 is applied and appname-0002 is not applied, and will
otherwise apply or unapply as few migrations as possible.
I also don't see a backwards-compatibility issue with switching to that
contract in 1.7.1. This is code that runs interactively and tells you
exactly what it is doing as it is doing it.
--
Ticket URL: <https://code.djangoproject.com/ticket/23474#comment:11>
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