неудачные миграции django с mysql - аргумент TypeError: int () должен быть строкой, байтовоподобным объектом или числом, а не 'CustomUser

Question

у нас был проект, работающий с sqlite, но когда мы попытались использовать mysql в качестве базы данных, мы столкнулись с этой проблемой:

    PS C:\GitHub\Javagochi\javagochi-server> python manage.py migrate
Operations to perform:
  Apply all migrations: account, admin, auth, authtoken, contenttypes, items, javagochi, sessions, sites, trades, users
Running migrations:
  Applying javagochi.0003_auto_20190330_1208...Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\core\management\commands\migrate.py", line 203, in handle
    fake_initial=fake_initial,
  File "C:\Program Files\Python37\lib\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 "C:\Program Files\Python37\lib\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 "C:\Program Files\Python37\lib\site-packages\django\db\migrations\executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\Program Files\Python37\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "C:\Program Files\Python37\lib\site-packages\django\db\migrations\operations\fields.py", line 216, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "C:\Program Files\Python37\lib\site-packages\django\db\backends\base\schema.py", line 523, in alter_field
    old_db_params, new_db_params, strict)
  File "C:\Program Files\Python37\lib\site-packages\django\db\backends\base\schema.py", line 627, in _alter_field
    new_default = self.effective_default(new_field)
  File "C:\Program Files\Python37\lib\site-packages\django\db\backends\base\schema.py", line 239, in effective_default
    return field.get_db_prep_save(default, self.connection)
  File "C:\Program Files\Python37\lib\site-packages\django\db\models\fields\related.py", line 937, in get_db_prep_save
    return self.target_field.get_db_prep_save(value, connection=connection)
  File "C:\Program Files\Python37\lib\site-packages\django\db\models\fields\__init__.py", line 790, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File "C:\Program Files\Python37\lib\site-packages\django\db\models\fields\__init__.py", line 956, in get_db_prep_value
    value = self.get_prep_value(value)
  File "C:\Program Files\Python37\lib\site-packages\django\db\models\fields\__init__.py", line 965, in get_prep_value
    return int(value)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'CustomUser'

в качестве внешнего ключа (мы думаем) в этом классе модели

class Javagochi(models.Model):
race = models.ForeignKey(JavagochiBase, on_delete=models.CASCADE, blank=False)
owner = models.ForeignKey(CustomUser, on_delete=models.CASCADE, blank=False, db_column='user__id')

nickname = models.CharField(max_length=255)
current_health = models.IntegerField()
current_hunger = models.IntegerField()
current_cold = models.IntegerField()
current_hot = models.IntegerField()
current_age = models.IntegerField(default=1)
current_level = models.IntegerField(default=1)
current_experience = models.IntegerField(default=0)

def __str__(self):
    return self.nickname + ', ' + self.race.race

и это наш пользовательский пользователь

class CustomUser(AbstractUser):
username = models.CharField(max_length=255, primary_key=True)
level = models.IntegerField(default = 1)
exp = models.IntegerField(default = 0)
coins = models.IntegerField(default = 1000)
# image = models.ImageField(upload_to='profile_image', blank=True)

def __str__(self):
    return self.username

мы думаем, что проблема в том, что django пытается использовать объект CustomUser в качестве внешнего ключа (вместо его идентификатора), но, похоже, не можетреши это.