#36129: Geodjango migrations ignore change of dimension in MultiPointField
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Reporter: Paweł Kotiuk | Owner: (none)
Type: Bug | Status: closed
Component: GIS | Version: dev
Severity: Normal | Resolution: invalid
Keywords: migrations | Triage Stage: Unreviewed
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 Paweł Kotiuk):
Hello Natalia, thank you for quick response.
Originally I discovered this bug, when I wanted to decrease number of
dimensions, and I was surprised that every single unit test in my test
suite failed. And generated migration (even for database generated from
scratch) always fails with: `ValueError: Cannot alter field...`
> Can you specify what backend are you using?
My database backend is `django.contrib.gis.db.backends.spatialite`
> in that how the added dimension would work with respect to the existing
values in the DB?
**For now migration breaks entire database, even when it is recreated from
scratch only for tests. **
I think it should work in the same way as migration for providing new
required value for model. So we should at least get a prompt asking about
default value we want to use.
> it seems quite challenging to generate a migration that makes sense for
the backend
In case of migration from 2 to 3 dimensions yes, but in case of migrating
from 3 dimensions to 2 it should be rather simple. Just delete Z axis.
**I think , that at least this single case should be somehow tackled (and
in other cases some warning should be shown).**
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I think this issue should be reopened, because it causes unexpected
problems with django project by generating broken migrations and it should
be somehow tackled. Either by providing reliable migration mechanism or at
least by showing warning during migration or generating migration files.
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Ticket URL: <https://code.djangoproject.com/ticket/36129#comment:3>
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