ValueError: Related model cannot be resolved in Django

Question

I'm using Django 2.x.

I have a model user_setting which have foreign Key to Country model which was created under address module.

Now, I'm using djanog-countries-plus to add Country data and thus have changed user_setting module to now point to Country model of django-countries-plus.

from countries_plus.models import Country
from django.contrib.auth.models import User

class UserSetting(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    country = models.ForeignKey(Country, on_delete=models.PROTECT, blank=True, null=True, default=None)
    modified = models.DateTimeField(auto_now=True)
    created = models.DateTimeField(auto_now_add=True)

The initial migration generated was

# 000_initial
class Migration(migrations.Migration):

    initial = True

    dependencies = [
        ('address', '0001_initial'),
        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
    ]

    operations = [
        migrations.CreateModel(
            name='UserSetting',
            fields=[
                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
                ('modified', models.DateTimeField(auto_now=True)),
                ('created', models.DateTimeField(auto_now_add=True)),
                ('country', models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='address.Country')),
                ('user', models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
            ],
            options={
                'db_table': 'user_setting',
            },
        ),
    ]

and the new migration generated after changing the reference of the ForeignKey field is

# 0002_auto_20181108_0724
class Migration(migrations.Migration):

    dependencies = [
        ('accounts', '0001_initial'),
    ]

    operations = [
        migrations.AlterField(
            model_name='usersetting',
            name='country',
            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='countries_plus.Country'),
        ),
    ]

The settings have installed apps like

'countries_plus',

#####################
# created apps
#####################
'accounts',

But when I run migrate, it gives error as

raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model 'countries_plus.Country' cannot be resolved

Full log

Running migrations:
  Applying accounts.0002_auto_20181108_0724...Traceback (most recent call last):
  File "src/manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 203, in handle
    fake_initial=fake_initial,
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 216, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 495, in alter_field
    new_db_params = new_field.db_parameters(connection=self.connection)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 966, in db_parameters
    return {"type": self.db_type(connection), "check": self.db_check(connection)}
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 963, in db_type
    return self.target_field.rel_db_type(connection=connection)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 878, in target_field
    return self.foreign_related_fields[0]
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 632, in foreign_related_fields
    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 619, in related_fields
    self._related_fields = self.resolve_related_fields()
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/fields/related.py", line 604, in resolve_related_fields
    raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model 'countries_plus.Country' cannot be resolved

The migration fails in 0002_auto_20181108_0724.

I have to make this change to already running application on Heroku.

Answer 1

This is a little bit a guess, but try to add dependency to migration:

# 0002_auto_20181108_0724
class Migration(migrations.Migration):

    dependencies = [
        ('accounts', '0001_initial'),
        ('countries_plus', '__latest__'),
    ]

    operations = [
        migrations.AlterField(
            model_name='usersetting',
            name='country',
            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='countries_plus.Country'),
        ),
    ]

Answer 2

For me the solution was to delete the table Countries and add it to the same migration file. If this is an option for you it will fix your problem. Maybe you can have a backup of that table and restore it after you fix the problem.

Something like this:

  # 0002_auto_20181108_0724
        class Migration(migrations.Migration):
        
            dependencies = [
                ('countries_plus', '0001_initial'),
            ]
            
            operations = [
                migrations.CreateModel(
                    name='Countries',
                    fields=[
                        (...),
                    ],
            ),
                migrations.AlterField(
                    model_name='usersetting',
                    name='country',
                    field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='countries_plus.Country'),
                ),
            ]