Adding an AutoField prompts for a default which creates a broken migration

[Tim Graham]

Change:

class Foo(models.Model):
    id = models.CharField(max_length=20, primary_key=True)

to:

class Foo(models.Model):
    character_id = models.AutoField(primary_key=True)

The created migration has these operations:

migrations.RemoveField(
    model_name='foo',
    name='id',
),
migrations.AddField(
    model_name='foo',
    name='character_id',
    field=models.AutoField(default=0, primary_key=True, serialize=False),
    preserve_default=False,
),

(entering 0 at the default prompt). Running the migration on PostgreSQL gives: ProgrammingError: multiple default values specified for column "character_id" of table "t27267_foo". I'm not sure if this can be made to work sensibly. Removing the default from the migration operation works, but if there's an existing data, the character_id field must be populated somehow (perhaps a RunPython could be done in the same migration?). #27338 is related. This was discovered while investigating #27267.

[Akshay Raj Gollahalli]

In Django 2.0b1 when trying to migrate without the default value, the CLI asks to enter a default value.

[za]

I am facing the same issue, but on my case I don't have data on the existing table. I'll think how to solve this issue.