Removing blank=True, null=True from ManyToMany field causes data deletion in migration

[Mark Tranchant]

My app has a Group model with an m2m field in it which used to look like:

things = models.ManyToManyField(Thing, blank=True, null=True)

Django 1.8a1 on my test environment warned me that the blank and null arguments were unnecessary, so I removed them:

things = models.ManyToManyField(Thing)

Then I ran manage.py makemigrations and applied the resulting migration with manage.py migrate. To my horror, the data in the relevant "linking" table was all deleted.

The section from the migration file looks like this:

    operations = [
        migrations.RemoveField(
            model_name='group',
            name='things',
        ),
        migrations.AddField(
            model_name='group',
            name='things',
            field=models.ManyToManyField(to='sa.Thing'),
        ),
    ]

This doesn't look like the right behaviour to me, and cost me a lot of time recovering data from backup. Have I done something wrong (other than trusting the migration file without inspecting it) or is this a genuine bug?

[Tim Graham]

Seems to be a regression in 1.8 as 1.7 creates a single AlterField operation.

[Tim Graham]

Bisected the regression to 623ccdd598625591d1a12fc1564cf3ef9a87581f. Attaching a regression test.

[Markus Holtermann]

PR: ​https://github.com/django/django/pull/4233

[Markus Holtermann <info@…>]

In a9e29fae105d1ddd4e0ac2059cbe62b0ee348bc8:

Fixed #24435 -- Prevented m2m field removal and addition in migrations when changing blank

Thanks Mark Tranchant for the report an Tim Graham for the test and
review.

[Markus Holtermann <info@…>]

In bff446c205e0c4d436a5147906397ab7534343f3:

[1.8.x] Fixed #24435 -- Prevented m2m field removal and addition in migrations when changing blank

Thanks Mark Tranchant for the report and Tim Graham for the test and
review.

Backport of a9e29fae105d1ddd4e0ac2059cbe62b0ee348bc8 from master