Missing import statement in generated migration (NameError: name 'models' is not defined)

[Jaap Joris Vens]

I found a bug in Django's latest release: 3.2.4.

Given the following contents of models.py:

from django.db import models

class MyField(models.TextField):
    pass

class MyBaseModel(models.Model):
    class Meta:
        abstract = True

class MyMixin:
    pass

class MyModel(MyMixin, MyBaseModel):
    name = MyField(primary_key=True)

The makemigrations command will generate the following migration file:

# Generated by Django 3.2.4 on 2021-06-30 19:13

import app.models
from django.db import migrations


class Migration(migrations.Migration):

    initial = True

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='MyModel',
            fields=[
                ('name', app.models.MyField(primary_key=True, serialize=False)),
            ],
            options={
                'abstract': False,
            },
            bases=(app.models.MyMixin, models.Model),
        ),
    ]

Which will then fail with the following error:

  File "/home/jj/django_example/app/migrations/0001_initial.py", line 7, in <module>
    class Migration(migrations.Migration):
  File "/home/jj/django_example/app/migrations/0001_initial.py", line 23, in Migration
    bases=(app.models.MyMixin, models.Model),
NameError: name 'models' is not defined

Expected behavior: Django generates a migration file that is valid Python.

Actual behavior: Django generates a migration file that is missing an import statement.

I think this is a bug of the module django.db.migrations.writer, but I'm not sure. I will be happy to assist with debugging.

Thanks for your attention,
Jaap Joris

[Abhyudai]

I could reproduce the issue with 3.2.4, 2.2.24 and the main branch. For what it's worth, the issue doesn't occur if the class MyModel does inherit from MyMixin.

[Mariusz Felisiak]

MyBaseModel is not necessary to reproduce this issue, it's due to the fact that MyModel doesn't have fields from django.db.models and has custom bases. It looks like an issue with special casing of models.Model in TypeSerializer. Proposed patch

diff --git a/django/db/migrations/serializer.py b/django/db/migrations/serializer.py
index e19c881cda..6e78462e95 100644
--- a/django/db/migrations/serializer.py
+++ b/django/db/migrations/serializer.py
@@ -273,7 +273,7 @@ class TupleSerializer(BaseSequenceSerializer):
 class TypeSerializer(BaseSerializer):
     def serialize(self):
         special_cases = [
-            (models.Model, "models.Model", []),
+            (models.Model, "models.Model", ['from django.db import models']),
             (type(None), 'type(None)', []),
         ]
         for case, string, imports in special_cases:

[Abhyudai]

​pull request

[Mariusz Felisiak <felisiak.mariusz@…>]

In fde6fb2:

Fixed #32893 -- Fixed serialization of models.Model class in migrations.

Migrations assumed that an import of the models.Model class must
already be included when it's serialized, but for models with only
custom fields this was not necessarily the case.

Thanks Jaap Joris Vens for the report.