Using of JoinManager leads to RuntimeError

[mark-mishyn]

JoinManager "raises Model class model_utils.managers.TempModel doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS"

Environment

• Django Model Utils version: 4.0.0
• Django version: 2.2.9
• Python version: 3.6.7
• Other libraries used, if any: pip freeze shows more than 150 libs

Code examples

Message.objects.all()[0:20].join()
RuntimeError: Model class model_utils.managers.TempModel doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

Possible solutions:

1. Documentation update: always require to add "model_utils" to INSTALLED_APP OR
2. Documentation update: add note to JoinManager, that it's usage requires "model_utils" in INSTALLED_APP OR
3. Explicitly add TempModel.Meta.app_label = 'model_utils' in model_utils/managers.py

P. S. I hope it will be useful for someone with the same issue