[Django] #26794: Migrating ForeignKey to OneToOneField creates extra index on PostgreSQL

[Django]

#26794: Migrating ForeignKey to OneToOneField creates extra index on PostgreSQL
----------------------------+--------------------
     Reporter:  jdufresne   |      Owner:  nobody
         Type:  Bug         |     Status:  new
    Component:  Migrations  |    Version:  master
     Severity:  Normal      |   Keywords:
 Triage Stage:  Unreviewed  |  Has patch:  0
Easy pickings:  0           |      UI/UX:  0
----------------------------+--------------------
 Start with the models:

 {{{
 class A(models.Model):
     pass

 class B(models.Model):
     orig_onetoone = models.OneToOneField(A, on_delete=models.PROTECT,
 related_name='+')
     orig_fk = models.ForeignKey(A, on_delete=models.PROTECT,
 related_name='+')
 }}}

 Run `makemigrations`, then change `orig_fk` to a `OneToOneField`:

 {{{
 class A(models.Model):
     pass

 class B(models.Model):
     orig_onetoone = models.OneToOneField(A, on_delete=models.PROTECT,
 related_name='+')
     orig_fk = models.OneToOneField(A, on_delete=models.PROTECT,
 related_name='+')
 }}}

 Run `makemigrations` again.

 Now run `migrate` to create the database tables. The table created for
 model `B`:

 {{{
 \d testapp_b
                               Table "public.testapp_b"
       Column      |  Type   |                       Modifiers
 
------------------+---------+--------------------------------------------------------
  id               | integer | not null default
 nextval('testapp_b_id_seq'::regclass)
  orig_fk_id       | integer | not null
  orig_onetoone_id | integer | not null
 Indexes:
     "testapp_b_pkey" PRIMARY KEY, btree (id)
     "testapp_b_orig_fk_id_68b3dcb4_uniq" UNIQUE CONSTRAINT, btree
 (orig_fk_id)
     "testapp_b_orig_onetoone_id_key" UNIQUE CONSTRAINT, btree
 (orig_onetoone_id)
     "testapp_b_ca8b5791" btree (orig_fk_id)
 Foreign-key constraints:
     "testapp_b_orig_fk_id_68b3dcb4_fk_testapp_a_id" FOREIGN KEY
 (orig_fk_id) REFERENCES testapp_a(id) DEFERRABLE INITIALLY DEFERRED
     "testapp_b_orig_onetoone_id_2353312c_fk_testapp_a_id" FOREIGN KEY
 (orig_onetoone_id) REFERENCES testapp_a(id) DEFERRABLE INITIALLY DEFERRED
 }}}

 Notice the column `orig_fk` has two indexes while the column
 `orig_onetoone` has only a single index.

 As these two columns are now described in the Python model identically, I
 expect them to have the same number of indexes.

 I created a test project with all the necessary migrations and models used
 to produce these results: https://github.com/jdufresne/djtest-fk-to-1to1

 Migrations:

 {{{
 0001

 class Migration(migrations.Migration):

     initial = True

     dependencies = [
     ]

     operations = [
         migrations.CreateModel(
             name='A',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
             ],
         ),
         migrations.CreateModel(
             name='B',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
                 ('orig_fk',
 models.ForeignKey(on_delete=django.db.models.deletion.PROTECT,
 related_name='+', to='testapp.A')),
                 ('orig_onetoone',
 models.OneToOneField(on_delete=django.db.models.deletion.PROTECT,
 related_name='+', to='testapp.A')),
             ],
         ),
     ]


 0002

 class Migration(migrations.Migration):

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

     operations = [
         migrations.AlterField(
             model_name='b',
             name='orig_fk',
 field=models.OneToOneField(on_delete=django.db.models.deletion.PROTECT,
 related_name='+', to='testapp.A'),
         ),
     ]
 }}}

--
Ticket URL: <https://code.djangoproject.com/ticket/26794>
Django <https://code.djangoproject.com/>
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