#34417: AlterField migration on ForeignKey field re-creates foreign key
constraints
unnecessarily
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Reporter: Ömer Faruk Abacı | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Migrations | Version: 4.1
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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Changes (by Simon Charette):
* stage: Unreviewed => Accepted
Comment:
Dropping the foreign key and recreating it [https://dbfiddle.uk/hgRUL6q6
is effectively not necessary on PostgreSQL] but
[https://dbfiddle.uk/OJ6KqwrW it is on MySQL] as this backend requires an
index on the ''referencing'' side of the relation and will silently
default to using an existing one if its present (see #31335 for more
details).
Given the contention issues associated with dropping a foreign key on
PostgreSQL I think it's a worthy investment to try to make things better
on databases that are not affected by this quirk.
Would you like
[https://docs.djangoproject.com/en/4.1/internals/contributing/writing-code
/submitting-patches/ to work on a patch]? The
[https://github.com/django/django/blob/d2b688b966f5d30414899549412d370e1317ddb8/django/db/backends/base/schema.py#L852-L1198
alter_field method is where you want to start looking] and you'll likely
need a feature flag to denote that foreign keys must be dropped or not
based on the backend.
--
Ticket URL: <https://code.djangoproject.com/ticket/34417#comment:1>
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