RuntimeError: Model class django_messages.models.Message doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS

I am trying to use https://github.com/arneb/django-messages package for my messaging stuff and tried the following

pip install git+https://github.com/arneb/django-messages.git

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'django.contrib.sites',
    'rest_framework',
    'rest_framework.authtoken',
    'oauth2_provider',
    'api',
    'localflavor',
    'django_localflavor_us',
    'django_countries',
    'haystack',
    'django_messages',

]

urlpatterns = [
    url(r'^admin/', admin.site.urls),
    url(r'^messages/', include('django_messages.urls')),
]

and when I run python manage.py runserver I got the following error:

Unhandled exception in thread started by <function wrapper at 0x107cedaa0>
Traceback (most recent call last):
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 121, in inner_run
    self.check(display_num_errors=True)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/management/base.py", line 374, in check
    include_deployment_checks=include_deployment_checks,
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/management/base.py", line 361, in _run_checks
    return checks.run_checks(**kwargs)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/checks/registry.py", line 81, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/checks/urls.py", line 14, in check_url_config
    return check_resolver(resolver)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/core/checks/urls.py", line 24, in check_resolver
    for pattern in resolver.url_patterns:
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/utils/functional.py", line 35, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/urls/resolvers.py", line 313, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/utils/functional.py", line 35, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/urls/resolvers.py", line 306, in urlconf_module
    return import_module(self.urlconf_name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/Users/shivakrishna/shiva/Office_projects/iPitch/project /project /urls.py", line 23, in <module>
    url(r'^messages/', include('django_messages.urls')),
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/conf/urls/__init__.py", line 50, in include
    urlconf_module = import_module(urlconf_module)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django_messages/urls.py", line 4, in <module>
    from django_messages.views import *
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django_messages/views.py", line 11, in <module>
    from django_messages.models import Message
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django_messages/models.py", line 48, in <module>
    class Message(models.Model):
  File "/Users/shivakrishna/.virtualenvs/project /lib/python2.7/site-packages/django/db/models/base.py", line 113, in __new__
    "INSTALLED_APPS." % (module, name)
RuntimeError: Model class django_messages.models.Message doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

Can I know the reason why that error is happening and how to solve it?

Solution

As stated by the answers to this questions:

The error you are getting is related to using a model before it gets initialized by the application.

You are stating in your comment that you are not using signals that execute at initialization, so search for:

Any method called in initialization and using your models or the django-messages models. If you have any, try to place those method calls inside the MyAppConfig class in your app.py.

If you have any relative path in your execution, change it to the exact module path:

from .models import MyModel
from .messages import *

to

from apps.myapp.models import MyModel
django.messages import *