Cannot migrate Django from 1.7 to 1.8

Question

When upgrading django 1.7 to 1.8 (final goal is migrating 1.4 to 1.11LTS) I am asked to remove SOUTH form my INSTALLED_APPS

Fine, but then I get:

raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet. 

So I tried to add in settings file:

import django
django.setup()

but it brings this error at runtime, that make me think it is not the proper way:

AUTH_USER_MODEL refers to model 'auth.User' that has not been installed

my INSTALLED_APPS:

'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.staticfiles',
'mptt',
# 'south',
'main',
'proxy',
'picocms',
'django.contrib.admin',
'reports',
'django_orphaned'

Note that if I comment 'main' (my main code folder) and 'picocms' (an outdated lib), the AppRegistryNotReady exception is not raised anymore(but obviously the project cannot work without it)

Does someone successfully went through the same kind of issue?

stackTrace (for info)

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/core/management/__init__.py", line 312, in execute
    django.setup()
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/local/Cellar/python@2/2.7.15_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File ".../workspace/django/cms/main/models.py", line 19, in <module>
    from picocms.models import CMSModel, CMSCategory, ActiveModelManager, PublicModelManager
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/picocms/models.py", line 13, in <module>
    class CMSCategory(MPTTModel):
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/mptt/models.py", line 189, in __new__
    return meta.register(cls)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/mptt/models.py", line 273, in register
    manager.init_from_model(cls)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/mptt/managers.py", line 58, in init_from_model
    [tree_field] = [fld for fld in model._meta.get_fields_with_model() if fld[0].name == self.tree_id_attr]
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 56, in wrapper
    return fn(*args, **kwargs)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 432, in get_fields_with_model
    return [self._map_model(f) for f in self.get_fields()]
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 740, in get_fields
    return self._get_fields(include_parents=include_parents, include_hidden=include_hidden)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 802, in _get_fields
    all_fields = self._relation_tree
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/utils/functional.py", line 60, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 709, in _relation_tree
    return self._populate_directed_relation_graph()
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/db/models/options.py", line 681, in _populate_directed_relation_graph
    all_models = self.apps.get_models(include_auto_created=True)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/utils/lru_cache.py", line 101, in wrapper
    result = user_function(*args, **kwds)
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/apps/registry.py", line 168, in get_models
    self.check_models_ready()
  File ".../.virtualenvs/migre/lib/python2.7/site-packages/django/apps/registry.py", line 131, in check_models_ready
    raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

Answer 1

I upgraded the picocms deprecated project and switched mptt version to 0.9.0 and it worked.

But indeed, my time would be better spent writing a new version of the project in python 3 and Django 2.2