Migrations do not add a FK constraint for an existing column

[Jean-Louis Fuchs]

Pull request: ​https://github.com/django/django/pull/4235

Consider following model:

class ForeignKeyTest(models.Model):
    id = models.AutoField(primary_key=True)
    customer = models.IntegerField()

class Customer(models.Model):
    id = models.AutoField(primary_key=True)

Then it gets migrated to this:

class ForeignKeyTest(models.Model):
    id = models.AutoField(primary_key=True)
    customer = models.ForeignKey('Customer')

class Customer(models.Model):
    id = models.AutoField(primary_key=True)

The second migration won't create the foreign key. This does not fail with sqlite! Because these migrations are handled differently. It will fail with MySQL and probably Postgres too.

The code didn't detect this case: if old_field.rel doesn't exist alter_field() must always create the foreign key and ONLY if .rel exists it must check .db_constraint, too. Since no .rel also means there was no foreign key before.

[Jean-Louis Fuchs]

Removed redundant information and chose clearer title.

[Markus Holtermann]

RFC when remaining style update is done

[Markus Holtermann <info@…>]

In f4f0060feaee6bbd76a0d575487682bc541111e4:

Fixed #24447 -- Made migrations add FK constraints for existing columns

When altering from e.g. an IntegerField to a ForeignKey, Django didn't
add a constraint.

[Markus Holtermann <info@…>]

In 1ae2df6bfc3afadd7edb29a5887711f70cdb52c6:

[1.8.x] Fixed #24447 -- Made migrations add FK constraints for existing columns

When altering from e.g. an IntegerField to a ForeignKey, Django didn't
add a constraint.

Backport of f4f0060feaee6bbd76a0d575487682bc541111e4 from master

[Markus Holtermann <info@…>]

In 283b630d6324c936cbe42d8f0169f056b3ba59c6:

Fixed #24447 -- Made migrations add FK constraints for existing columns

When altering from e.g. an IntegerField to a ForeignKey, Django didn't
add a constraint.

Backport of f4f0060feaee6bbd76a0d575487682bc541111e4 from master