[Django] #32386: ForeignKey index is removed upon creation of a UniqueConstraint, which breaks migrating backwards

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Jan 26, 2021, 10:23:08 AM1/26/21

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#32386: ForeignKey index is removed upon creation of a UniqueConstraint, which
breaks migrating backwards
-------------------------------------+-------------------------------------
Reporter: Tijmen | Owner: nobody
Type: Bug | Status: new
Component: | Version: 3.1
Migrations | Keywords: backward migration
Severity: Normal | unique constraint foreignkey index
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
When adding a UniqueConstraint on a ForeignKey field combined with another
field, the original db index on the ForeignKey is removed and replaced by
the unique constraint.
This crashes the migration when trying to migrate backwards, because the
foreignkey needs to have an index.

{{{
django.db.utils.OperationalError: (1553, "Cannot drop index
'modelb_unique_position': needed in a foreign key constraint")
}}}

It makes no difference whether the unique constraint is added in the same
or a separate migration.
In the example below it's separated into 2 different migrations.

Expect behavior would be either for the index not be dropped when creating
the unique constraint, or for the index to be added before removing the
unique constraint when migrating backwards.

First migration:

{{{
from django.db import migrations, models
import django.db.models.deletion

class Migration(migrations.Migration):

dependencies = [
('appname', '0001_initial'),
]

operations = [
migrations.CreateModel(
name='ModelA',
fields=[
('id', models.AutoField(
auto_created=True,
primary_key=True,
serialize=False,
verbose_name='ID')),
],
),
migrations.CreateModel(
name='ModelB',
fields=[
('id', models.AutoField(
auto_created=True,
primary_key=True,
serialize=False,
verbose_name='ID')),
('a_instance', models.ForeignKey(
on_delete=django.db.models.deletion.PROTECT,
related_name='bees',
to='appname.modela',
verbose_name=('modela',))),
('position', models.IntegerField(
default=0,
null=True,
verbose_name='volgorde')),
],
),
]

}}}

second migration:

{{{
from django.db import migrations, models
import django.db.models.deletion

class Migration(migrations.Migration):

dependencies = [
('appname', '0002_modela_modelb'),
]

operations = [
migrations.AddConstraint(
model_name='modelb',
constraint=models.UniqueConstraint(
fields=('a_instance', 'position'),
name='modelb_unique_position'),
),
]

}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/32386>
Django <https://code.djangoproject.com/>
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Django

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Jan 26, 2021, 2:02:20 PM1/26/21

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Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Simon Charette):

* status: new => closed
* resolution: => duplicate

Comment:

I assume you are using MySQL, if it's the case then that's a duplicate of
#31335, see https://code.djangoproject.com/ticket/#comment:2.