Migrations don't alter foreign key data types when referencing primary keys in MTI models.

[Mariusz Felisiak]

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.

[David Wobrock]

PR: ​https://github.com/django/django/pull/14691

[Mariusz Felisiak <felisiak.mariusz@…>]

In 325d7710:

Fixed #32743 -- Added foreign key altering when altering type of referenced primary key with MTI.

[Keryn Knight]

There's an additional PR which been made which alludes to a situation missed by the original fix: ​https://github.com/django/django/pull/14700
I can't speak for the veracity of it as I haven't been following the details of the ticket, but it's probably worth someone glancing at it and rolling it into this ticket's fix log if appropriate.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3d9040a5:

Refs #32743 -- Fixed recreation of foreign key constraints when altering type of referenced primary key with MTI.

Follow up to 325d7710ce9f6155bb55610ad6b4580d31263557.