makemigrations produces invalid import path in migration file

[Choong Jun Jin]

This issue is somewhat similar to https://code.djangoproject.com/ticket/27914 with a slightly different problem.

If you define an inner class in the django.db.models.Model subclass, makemigrations will produce an invalid import path resulting in import error.

Specifically, if we have the following structure in our models.py,

from django.db import models


class OuterClass(models.Model):
    class InnerField(models.CharField):
        pass

    outer_field = InnerField(max_length=100)

the generated migration file would produce

# Generated by Django 3.2.6 on 2021-08-13 03:39

from django.db import migrations, models
import test1.models.OuterClass


class Migration(migrations.Migration):

    initial = True

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='OuterClass',
            fields=[
                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('outer_field', test1.models.OuterClass.InnerField(max_length=100)),
            ],
        ),
    ]

Notice that the import path at the top refers to an import of a class. This is an invalid import format.

import test1.models.OuterClass

Further usage of manage.py migrate will result in errors:

ModuleNotFoundError: No module named 'test1.models.OuterClass'; 'test1.models' is not a package

If we change the import to

import test1.models

the migration file will work accordingly.

[Carlton Gibson]

You'll need to ​register a custom serializer that knows how to handle your nested field. (A nice example is ​`DatetimeDatetimeSerializer` which adjusts to the `tzinfo` to `django.utils.timezone.utc`, rather than the underlying implmentation.)