Altering a 1to1 field to FK on Postgres drops uniqueness but does not create index

[synotna]

Changing a OneToOneField to a ForeignKey does not trigger an index to be created by the migration

        migrations.AlterField(
            model_name='contracttwitchchannelrevenueadsbycountry',
            name='revenue_data',
            field=models.ForeignKey(to='twitchrevenue.TwitchRevenueDataAdsByCountry'),
            preserve_default=True,
        ),

SQL for the migration:

 ./manage.py sqlmigrate partners 0021

BEGIN;
ALTER TABLE "partners_contracttwitchchannelrevenueadsbycountry" DROP CONSTRAINT "a45496b32304e658c2a94795584a1889";
ALTER TABLE "partners_contracttwitchchannelrevenueadsbycountry" ADD CONSTRAINT "D321aaa5509069df3ee6b51c0eb97103" FOREIGN KEY ("revenue_data_id") REFERENCES "twitchrevenue_twitchrevenuedataadsbycountry" ("id") DEFERRABLE INITIALLY DEFERRED;

COMMIT;

Also, explicitly stating db_index=True does not cause a new migration to be created

[Shai Berger]

Why should one be created? If it was a OneToOneField, it should already have an index, and I see nothing there to drop it.

I suppose you came to the conclusion that something was wrong because something failed for you; it would be more useful to describe the problem you encountered and the process that led you to the conclusion that the behavior was wrong (but, the proper place to do so would be a users forum like the ​django-users group).

I am closing the ticket as invalid based on the info given; feel free to reopen (and explain) if I've misunderstood, or to open a separate ticket if appropriate.

[synotna]

Sorry, you are correct: I did not make the connection that the OneToOneField should have already created the index beforehand

Here is the migration that created the model including the OneToOneField, and the output from sqlmigrate which shows that index creation was missing:

        migrations.CreateModel(
            name='ContractTwitchChannelRevenueAdsByCountry',
            fields=[
                ('id', models.AutoField(primary_key=True, serialize=False, auto_created=True, verbose_name='ID')),
                ('partner_revenue', models.DecimalField(decimal_places=7, max_digits=12)),
                ('contract_twitch_channel', models.ForeignKey(to='partners.ContractTwitchChannel')),
                ('revenue_data', models.OneToOneField(to='twitchrevenue.TwitchRevenueDataAdsByCountry')),
            ],
            options={
            },
            bases=(models.Model,),
        ),

./manage.py sqlmigrate partners 0007

BEGIN;
CREATE TABLE "partners_contracttwitchchannelrevenueadsbycountry" ("id" serial NOT NULL PRIMARY KEY, "partner_revenue" numeric(12, 7) NOT NULL, "contract_twitch_channel_id" integer NOT NULL, "revenue_data_id" integer NOT NULL UNIQUE);
ALTER TABLE "partners_contracttwitchchannelrevenueadsbycountry" ADD CONSTRAINT "ccbec8ea2680b0e44e335452dd041304" FOREIGN KEY ("contract_twitch_channel_id") REFERENCES "partners_contracttwitchchannel" ("id") DEFERRABLE INITIALLY DEFERRED;
ALTER TABLE "partners_contracttwitchchannelrevenueadsbycountry" ADD CONSTRAINT "a1dfe1f83ea9a2dbb28dc2ec53ef0136" FOREIGN KEY ("revenue_data_id") REFERENCES "twitchrevenue_twitchrevenuedataadsbycountry" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "partners_contracttwitchchannelrevenueadsbycountry_71b8390c" ON "partners_contracttwitchchannelrevenueadsbycountry" ("contract_twitch_channel_id");

COMMIT;

Am I correct in thinking that is a bug? If so then I'll edit (or open) a new bug ticket correctly

[Tim Graham]

My understanding is that an index ​shouldn't be created in this case since OneToOneField is unique=True: "Note that when unique is True, you don’t need to specify db_index, because unique implies the creation of an index."

[Shai Berger]

Ok, I see now. The PG backend relies on the unique constraint to create an index (correctly) for a 1-1 field; which means, if the field is no longer unique but still a FK, it needs an explicit index.

(you can probably work around this by adding a single-field index_together entry to your Meta)

[Tim Graham]

See also the opposite case of ForeignKey to OneToOneField in #26090.

[Jon Dufresne]

​https://github.com/django/django/pull/6851

[Tim Graham <timograham@…>]

In 9356f63a:

Fixed #25317, #26090 -- Fixed swapping combinations of unique and db_index during migrations.