avoid changing relations from unmanaged models

[Felix Engelmann]

The main issue was applying old migrations of models that inherit but are unmanaged. A prime example is a database view that extends a real model, both in terms of python functions and with database columns.

The model.py is as follows:

from django.db import models

class Parent(models.Model):
    name = models.CharField()

class Child(Parent):
    class Meta:
        managed = False

The main culprit happened, when the DEFAULT_AUTO_FIELD setting changed e.g. from django.db.models.AutoField to BigAutoField. This lead makemigrations to create an operation:

migrations.AlterField(
            model_name='parent',
            name='id',
            field=models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID'),
        ),

This migration in the current code tried to follow the parent_ptr OneToOne relation and update the foreign key in the Child table. As Child is a view, this failed on postgres.
The proposed fix is to exclude unmanaged models in the _is_relevant_relation function to make old migrations run.

This migration works well in django 3.