Migration Fail with MTI and Many2Many relation when switching PK type

[Philipp]

This is somewhat related to #32743 and #32742

Consider the following example:
A simple inheritance where the child model has a M2M relation to some other model:

class SomeModel(models.Model):
    some_field = models.IntegerField(default=5)


class BaseModel(models.Model):
    some_other_base_field = models.IntegerField(default=5)

    class Meta:
        abstract = False

class DerivedModel(BaseModel):
    relation = models.ManyToManyField(SomeModel, related_name='+')

I have altered migration 0001 manually to mimic the create behaviour prior to Django 3.2 (AutoField instead of BigAuto for PK)
The second migration correctly picks up that this needs to be changed.

The resulting sql query for 0002 is:

--
-- Alter field id on basemodel
--
ALTER TABLE `sample_derivedmodel` DROP FOREIGN KEY `sample_derivedmodel_basemodel_ptr_id_a3110b27_fk_sample_ba`;
ALTER TABLE `sample_basemodel` MODIFY `id` bigint AUTO_INCREMENT NOT NULL;
ALTER TABLE `sample_derivedmodel` MODIFY `basemodel_ptr_id` bigint NOT NULL; <------------- FAILS
ALTER TABLE `sample_derivedmodel` ADD CONSTRAINT `sample_derivedmodel_basemodel_ptr_id_a3110b27_fk` FOREIGN KEY (`basemodel_ptr_id`) REFERENCES `sample_basemodel` (`id`);
--
-- Alter field id on somemodel
--
ALTER TABLE `sample_derivedmodel_relation` DROP FOREIGN KEY `sample_derivedmodel__somemodel_id_31aa39d5_fk_sample_so`;
ALTER TABLE `sample_somemodel` MODIFY `id` bigint AUTO_INCREMENT NOT NULL;
ALTER TABLE `sample_derivedmodel_relation` MODIFY `somemodel_id` bigint NOT NULL;

The marked line fails with the following message:

Error Code: 3780. Referencing column 'derivedmodel_id' and referenced column 'basemodel_ptr_id' in foreign key constraint 'sample_derivedmodel__derivedmodel_id_9f9fbb1f_fk_sample_de' are incompatible.

And this FK constraint is the one in the sample_derivedmodel_relation table, I suspect because the relation table is not modified to use bigints for the foreign ids.

I observed this only when using MYSQL but not on SQLITE.

I prepared a minimal reproducible example here:
​https://github.com/Hafnernuss/MigrationFail

[Mariusz Felisiak]

Thanks for the report, it's fixed on Django 4.0+ in 325d7710ce9f6155bb55610ad6b4580d31263557.

Duplicate of #32743.