Initial migration fails when referencing a custom user model from a sequence of at least two concrete ancestor models

[Steven Ganz]

The following model fails on initial migration.

from django.db import models
from django.contrib.auth.models import (AbstractBaseUser)

class A(models.Model):
    createdByUser = models.ForeignKey('User', related_name='creations+', verbose_name='creatorUser', blank=True, null=True, editable=False, on_delete=models.PROTECT)

class B(A):
    pass

class User(AbstractBaseUser, B):
    email = models.EmailField('email address', max_length=256, unique=True, db_index=True)
    USERNAME_FIELD = 'email'

    @property
    def username(self):
         return self.email

settings.py contains:

AUTH_USER_MODEL = 'core.User'

  Applying core.0001_initial...Traceback (most recent call last):
  File "./manage.py", line 13, in <module>
    execute_from_command_line(sys.argv)
  File ".../lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File ".../lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ".../lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File ".../lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File ".../lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File ".../lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File ".../lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File ".../lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File ".../lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File ".../lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 88, in database_forwards
    field,
  File ".../lib/python3.6/site-packages/django/db/backends/base/schema.py", line 431, in add_field
    definition, params = self.column_sql(model, field, include_default=True)
  File ".../lib/python3.6/site-packages/django/db/backends/base/schema.py", line 160, in column_sql
    db_params = field.db_parameters(connection=self.connection)
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 994, in db_parameters
    return {"type": self.db_type(connection), "check": self.db_check(connection)}
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 991, in db_type
    return self.target_field.rel_db_type(connection=connection)
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 909, in target_field
    return self.foreign_related_fields[0]
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 653, in foreign_related_fields
    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 640, in related_fields
    self._related_fields = self.resolve_related_fields()
  File ".../lib/python3.6/site-packages/django/db/models/fields/related.py", line 625, in resolve_related_fields
    raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model 'core.User' cannot be resolved

Notably, this works fine with any of the following modifications:

• model B is removed, closing the chain
• createdByUser is moved to B or removed altogether
• either A or B is made abstract
• the referenced model is not the custom user model

This is not just a matter of preference for concrete classes, as only a concrete class can be referenced through a foreign key.