#25253: MySQL migrations drop & recreate constraints unnecessarily when changing
attributes that don't affect the schema
--------------------------------------+------------------------------------
Reporter: Thomas Recouvreux | Owner: Shun Yu
Type: Cleanup/optimization | Status: assigned
Component: Migrations | Version: 1.8
Severity: Normal | Resolution:
Keywords: migrations m2m mysql | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
--------------------------------------+------------------------------------
Comment (by israel-tk):
We had this problem with Postgres and Django 1.11 when deleting a not-null
constraint in a ForeignKey field (adding null=True to a field). Instead of
just dropping the not-null constraint which is a safe database operation,
it first dropped the ForeignKey constraint, then dropped the not-null
constraint, and last re-created the ForeignKey constraint (3 statements).
Re-creating ForeignKey constraints locks tables and can generate downtime.
Since we cannot afford the downtime we used a
[https://docs.djangoproject.com/en/2.1/ref/migration-operations/#runsql
RunSQL] migration operation with `state_operations` as a **workaround**:
{{{
migrations.RunSQL(
"""--
-- Alter field trip on payment
--
ALTER TABLE "app_payment" ALTER COLUMN "trip_id" DROP NOT NULL;
""",
state_operations=[
migrations.AlterField(
model_name='payment',
name='trip',
field=models.ForeignKey(blank=True, null=True,
on_delete=django.db.models.deletion.PROTECT,
related_name='payments',
to='app.Trip'),
),
])
}}}
This way we only apply the SQL we want and Django won't complain about the
migration state not matching the current model. :)
--
Ticket URL: <https://code.djangoproject.com/ticket/25253#comment:15>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/066.63ffc7e38421f28440deb0d822c12523%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.
