id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28972	`RelatedObjectDoesNotExist` during `loaddata` of models with multi-table inheritance	Kal Sze	nobody	"Software versions
* Python 3.6.3
* Django 2.0
* django-model-utils 3.1.1 (from git)
* PostgreSQL 9.4
* psycopg2 2.7.3.2

I have a `licenses` app, with these models:
* `BaseLicense`, a concrete model, with a `ForeignKey` to `settings.AUTH_USER_MODEL`.
* `CorelDrawLicense`, which inherits from `BaseLicense` through multi-table inheritance
* `AdobePhotoshopLicense`, which inherits from `BaseLicense` through multi-table inheritance

I created an instance of `AdobePhotoshopLicense`, then tried to do `manage.py dumpdata` and `manage.py loaddata`.

`dumpdata` ""worked"" - there was no error.

However, `loaddata` failed with this traceback:
{{{
Traceback (most recent call last):
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/serializers/json.py"", line 69, in Deserializer
    yield from PythonDeserializer(objects, **options)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/serializers/python.py"", line 170, in Deserializer
    obj = base.build_instance(Model, data, using)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/serializers/base.py"", line 225, in build_instance
    natural_key = obj.natural_key()
  File ""/home/kal/git/nk_test/licenses/models.py"", line 33, in natural_key
    return (self.user.natural_key(), self.license_key)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py"", line 175, in __get__
    ""%s has no %s."" % (self.field.model.__name__, self.field.name)
django.db.models.fields.related_descriptors.RelatedObjectDoesNotExist: BaseLicense has no user.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File ""./manage.py"", line 15, in <module>
    execute_from_command_line(sys.argv)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/__init__.py"", line 371, in execute_from_command_line
    utility.execute()
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/__init__.py"", line 365, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/base.py"", line 288, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/base.py"", line 335, in execute
    output = self.handle(*args, **options)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/commands/loaddata.py"", line 72, in handle
    self.loaddata(fixture_labels)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/commands/loaddata.py"", line 113, in loaddata
    self.load_label(fixture_label)
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/management/commands/loaddata.py"", line 168, in load_label
    for obj in objects:
  File ""/home/kal/.virtualenvs/nk_test-5iWsqJVv/lib/python3.6/site-packages/django/core/serializers/json.py"", line 73, in Deserializer
    raise DeserializationError() from exc
django.core.serializers.base.DeserializationError: Problem installing fixture '/home/kal/git/nk_test/./fixtures/dumpdata.json':
}}}

Here are the exact steps:
1. Create model instances in management shell
{{{
>>> from django.contrib.auth import get_user_model
>>> User = get_user_model()
>>> kal = User.objects.create_user('kal', 'kal@foobar.com', 'qwer1234')
>>> from licenses.models import AdobePhotoshopLicense
>>> l = AdobePhotoshopLicense.objects.create(user=kal, license_key='12345')
}}}

2. Dump the data into a fixtures json
{{{
$ python manage.py  dumpdata -o fixtures.json --format json --natural-foreign --natural-primary auth licenses
}}}

3. Edit settings.py to switch to an empty database
{{{
DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql',
        #'NAME': 'nk_test',
        'NAME': 'nk_test_2',
        'USER': 'kal',
        'HOST': '',
        'PORT': 5432,
    }
}
}}}

4. Run migrations on the new empty database
{{{
$ python manage.py migrate
}}}

5. Try to load the data from fixtures.json
{{{
$ python manage.py loaddata --format json fixtures.json
}}}

I have attached the `models.py` and `fixtures.json` files to the ticket.

I know that I don't (and I can't) enforce the natural key uniqueness at the database level. However, due to my application logic, I know that the user+license_key combination is effectively unique for any one of the child license models. And this is specifically allowed, [https://docs.djangoproject.com/en/2.0/topics/serialization/#deserialization-of-natural-keys according to the documentation]:

 [U]niqueness doesn’t need to be enforced at the database level. If you are certain that a set of fields will be effectively unique, you can still use those fields as a natural key.

Also note that this bug doesn't have to do with my use of the custom `InheritanceManager`. I could let the `BaseLicense` model use the default manager and still get the same bug."	Bug	closed	Core (Serialization)	2.0	Normal	duplicate			Unreviewed	0	0	0	0	0	0