Migrations call callable field defaults only once for all rows

[Rowan Seymour]

If you add a field with a callable default to an existing model, e.g.

def generate_uuid():
    return unicode(uuid4())

class Foo(models.Model):
    uuid = models.CharField(max_length=36, unique=True, default=generate_uuid)

You'll get a migration like...

migrations.AddField(
    model_name='foo',
    name='uuid',
    field=models.CharField(default=foo.models.generate_uuid, max_length=36, unique=True),
    preserve_default=True,
),

The default function is called only once to get a single default value for all rows, which in this case will fail because the column has a unique constraint.

[Tim Graham]

Duplicate of #23408