Warn about adding/dropping not null on foreign keys

[ljodal]

When Django alters anything on a foreign key column it will first drop the foreign key constraint, then make the schema changes, before finally re-adding the foreign key constraint. This requires an additional access exclusive lock that's not needed when just changing the nullability of the column.

For example this migration:

class Migration(migrations.Migration):

    operations = [
        migrations.AlterField(
            model_name='foo',
            name='field_name',
            field=models.ForeignKey(blank=True, on_delete=django.db.models.deletion.PROTECT, related_name='related_field_name', to='bar'),
        ),
    ]

Will generate this SQL:

BEGIN;
SET CONSTRAINTS "foo_field_name__f89f691c_fk_bar" IMMEDIATE; ALTER TABLE "foo" DROP CONSTRAINT "foo_field_name__f89f691c_fk_bar";
ALTER TABLE "foo" ALTER COLUMN "field_name_id" SET NOT NULL;
ALTER TABLE "foo" ADD CONSTRAINT "foo_field_name__f89f691c_fk_bar" FOREIGN KEY ("field_name_id") REFERENCES "bar" ("id") DEFERRABLE INITIALLY DEFERRED;
COMMIT;

However, just running the alter schema query will achieve the same and not lock the referenced table:

class Migration(migrations.Migration):

    operations = [
        migrations.RunSQL(
            'ALTER TABLE "foo" ALTER COLUMN "field_name_id" SET NOT NULL;',
            'ALTER TABLE "foo" ALTER COLUMN "field_name_id" DROP NOT NULL;',
            state_operations=[
                migrations.AlterField(
                    model_name='foo',
                    name='field_name',
                    field=models.ForeignKey(blank=True, on_delete=django.db.models.deletion.PROTECT, related_name='related_field_name', to='bar'),
                ),
             ],
         )
    ]