1.Django项目错误提示：
RuntimeError: Model class xxx.apps.users.models.User doesn’t declare an explicit app_label and isn’t in an application in INSTALLED_APPS.
2. 项目结构：
django_project
├── LICENSE
├── main.py
└── test_django
├── manage.py
└── test_django
├── apps
│ ├── app1
│ │ ├── admin.py
│ │ ├── apps.py
│ │ ├── __init__.py
│ │ ├── models.py
│ │ ├── tests.py
│ │ ├── urls.py
│ │ └── views.py
│ └── app2
│ ├── admin.py
│ ├── apps.py
│ ├── __init__.py
│ ├── models.py
│ ├── tests.py
│ ├── urls.py
│ └── views.py
├── asgi.py
├── __init__.py
├── settings
│ ├── dev.py
│ └── prod.py
├── urls.py
└── wsgi.py
2.解决
1. app内，使用相对路径引用Models模块：
# 在apps.app1.urls.py中导入apps.app1.views.py
from . import views
# 在apps.app1.views.py中导入apps.app1.models.py中的xxx
from .models import xxx
2. 不同app之间，使用以apps为根目录的绝对路径：
# 在apps.app2.views.py中导入apps.app1.models.py中的xxx
from app1.models import xxx
3. 其它参考
https://stackoverflow.com/questions/35388637/runtimeerror-model-class-django-contrib-sites-models-site-doesnt-declare-an-ex
