[Django] #32743: Migrations don't alter foreign key data types when referencing primary keys in MTI models.

[Django]

#32743: Migrations don't alter foreign key data types when referencing primary 
keys
in MTI models.
--------------------------------------------+------------------------
               Reporter:  Mariusz Felisiak  |          Owner:  nobody
                   Type:  Bug               |         Status:  new
              Component:  Migrations        |        Version:  3.2
               Severity:  Normal            |       Keywords:
           Triage Stage:  Unreviewed        |      Has patch:  0
    Needs documentation:  0                 |    Needs tests:  0
Patch needs improvement:  0                 |  Easy pickings:  0
                  UI/UX:  0                 |
--------------------------------------------+------------------------
 Migrations don't alter foreign key data types when referencing primary
 keys in MTI models, e.g.
 - before state:
 {{{
 class Parent(models.Model):
     pass


 class Child(Parent):
     pass


 class MainModel(models.Model):
     child = models.ForeignKey(Child, models.CASCADE)
 }}}
 - after state:
 {{{
 class Parent(models.Model):
     id = models.BigAutoField(primary_key=True)
 }}}

 `makemigrations` generates a single operation:
 {{{
 migrations.AlterField(
     model_name='parent',
     name='id',
     field=models.BigAutoField(primary_key=True, serialize=False),
 ),
 }}}
 which doesn't change `MainModel.child`'s datatype:
 {{{
 $ python manage.py sqlmigrate test_two 0002
 --
 -- Alter field id on parent
 --
 ALTER TABLE `test_two_child` DROP FOREIGN KEY
 `test_two_child_parent_ptr_id_537e8ba7_fk_test_two_parent_id`;
 ALTER TABLE `test_two_parent` MODIFY `id` bigint AUTO_INCREMENT NOT NULL;
 ALTER TABLE `test_two_child` MODIFY `parent_ptr_id` bigint NOT NULL;
 ALTER TABLE `test_two_child` ADD CONSTRAINT
 `test_two_child_parent_ptr_id_537e8ba7_fk` FOREIGN KEY (`parent_ptr_id`)
 REFERENCES `test_two_parent` (`id`);
 }}}

 and throws an error on MySQL:
 {{{
 MySQLdb._exceptions.OperationalError: (3780, "Referencing column
 'child_id' and referenced column 'parent_ptr_id' in foreign key constraint
 'test_two_mainmodel_child_id_3ca4683f_fk_test_two_' are incompatible.")
 }}}

 I couldn't find an existing ticket.

 Noticed when checking #32742.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/32743>
Django <https://code.djangoproject.com/>
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