#34417: AlterField migration on ForeignKey field re-creates foreign key
constraints
unnecessarily
------------------------------------------------+------------------------
Reporter: Ömer Faruk Abacı | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Migrations | Version: 4.1
Severity: Normal | Keywords:
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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Let's assume that we have the following models:
{{{
class Bar(models.Model):
pass
class Foo(models.Model):
bar = models.ForeignKey(Bar, related_name="foos",
on_delete=models.CASCADE)
}}}
The migration that creates these models will add an index to the `bar`
field of `Foo` model, this is well-documented and there is no problem up
until now. But when we do the following change to drop the index on the
ForeignKey field:
{{{
class Foo(models.Model):
bar = models.ForeignKey(..., db_index=False)
}}}
Then we have a migration that results in the following SQL statement:
{{{
BEGIN;
--
-- Alter field bar on foo
--
SET CONSTRAINTS "foos_foo_bar_id_efb062ad_fk_foos_bar_id" IMMEDIATE;
ALTER TABLE "foos_foo"
DROP CONSTRAINT "foos_foo_bar_id_efb062ad_fk_foos_bar_id";
DROP INDEX IF EXISTS "foos_foo_bar_id_efb062ad";
ALTER TABLE "foos_foo" ADD CONSTRAINT
"foos_foo_bar_id_efb062ad_fk_foos_bar_id"
FOREIGN KEY ("bar_id")
REFERENCES "bars_bar" ("id")
DEFERRABLE INITIALLY DEFERRED;
COMMIT;
}}}
I believe that we shouldn't be needing to re-create the FK constraint, so
the following SQL should suffice:
{{{
BEGIN;
--
-- Alter field bar on foo
--
DROP INDEX IF EXISTS "foos_foo_bar_id_efb062ad";
COMMIT;
}}}
Actually the main problem here is that dropping & adding constraints locks
the entire table, and it might be catastrophic for some live production
databases. IMHO this should either be documented or refactored. I look
forward to hear your opinions on this, thank you in advance!
--
Ticket URL: <https://code.djangoproject.com/ticket/34417>
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