Example celery v4.2 with mongodb results backend - python-3.x

Question

I am trying to setup a Celery application under Flask to accept API requests and then separate Celery workers to perform the long running tasks. My problem is that my Flask and everything else in my environment uses MongoDB so I do not want to setup a separate SQL db just for the Celery results. I cannot find any good examples of how to properly configure Celery with a MongoDB cluster as the backend.
Here are the settings I have tried to make it accept:
CELERY_RESULT_BACKEND = "mongodb"
CELERY_MONGODB_BACKEND_SETTINGS = {"host": "mongodb://mongodev:27017",
"database": "celery",
"taskmeta_collection": "celery_taskmeta"}
No matter what I do, Celery seems to ignore the config settings and launched without any results backend. Does anywon have a working example using the latest version of Celery? The only other examples I can find are of v3 Celery setups and that didn't work for me either since I am using a Mongo replica cluster in production which seems unsupported for that version.
[Edit]Adding more information in the complicated way I am setting the config to work with the rest of the application.
The config values are first passed as environment variables through a docker-compose file like this:
environment:
- PYTHONPATH=/usr/src/
- APP_SETTINGS=config.DevelopmentConfig
- FLASK_ENV=development
- CELERY_BROKER_URL=amqp://guest:guest#rabbit1:5672
- CELERY_BROKER_DEV=amqp://guest:guest#rabbit1:5672
- CELERY_RESULT_SERIALIZER=json
- CELERY_RESULT_BACKEND=mongodb
- CELERY_MONGODB_BACKEND_SETTINGS={"host":"mongodb://mongodev:27017","database":"celery","taskmeta_collection":"celery_taskmeta"}
Then, inside the config.py file they are loaded:
class DevelopmentConfig(BaseConfig):
"""Development configuration"""
CELERY_BROKER_URL = os.getenv('CELERY_BROKER_DEV')
CELERY_RESULT_SERIALIZER = os.getenv('CELERY_RESULT_SERIALIZER')
CELERY_RESULT_BACKEND = os.getenv('CELERY_RESULT_BACKEND')
CELERY_MONGODB_BACKEND_SETTINGS = ast.literal_eval(os.getenv('CELERY_MONGODB_BACKEND_SETTINGS'))
Then, when Celery is initiated, the config is loaded:
app = Celery('celeryworker', broker=os.getenv('CELERY_BROKER_URL'),
include=['celeryworker.tasks'])
print('app initiated')
app.config_from_object(app_settings)
app.conf.update(accept_content=['json'])
print("CELERY_MONGODB_BACKEND_SETTINGS",
os.getenv('CELERY_MONGODB_BACKEND_SETTINGS'))
print("celery config",app.conf)
When the application comes up here is what I see with all my troubleshooting prints. I have redacted a lot of the config output just to show what I have here is passing through the config.py to app.config but being ignored by Celery. You can see the value makes it into the celery.py file and I am sure Celery does something with it because before I added the ast.literal_eval in the config.py Celery would throw an error saying that the MongoDB backend settings needed to be a dict rather then a string. Unfortunately now that it is being passed as a proper dict Celery ignores it.
app_settings SGSDevOps.config.DevelopmentConfig
app initiated
CELERY_MONGODB_BACKEND_SETTINGS {"host":"mongodb://mongodev:27017","database":"celery","taskmeta_collection":"celery_taskmeta"}
celery config Settings(Settings({'BROKER_URL': 'amqp://guest:guest#rabbit1:5672', 'CELERY_INCLUDE': ['celeryworker.tasks'], 'CELERY_ACCEPT_CONTENT': ['json']}, 'BROKER_URL': 'amqp://guest:guest#rabbit1:5672', 'CELERY_MONGODB_BACKEND_SETTINGS': None, 'CELERY_RESULT_BACKEND': None}))
APP_SETTINGS config.DevelopmentConfig
app.config <Config {'ENV': 'development', 'CELERY_BROKER_URL': 'amqp://guest:guest#rabbit1:5672', 'CELERY_MONGODB_BACKEND_SETTINGS': {'host': 'mongodb://mongodev:27017', 'database': 'celery', 'taskmeta_collection': 'celery_taskmeta'}, 'CELERY_RESULT_BACKEND': 'mongodb', 'CELERY_RESULT_SERIALIZER': 'json', }>
-------------- celery#a5ea76b91f77 v4.2.1 (windowlicker)
---- **** -----
--- * *** * -- Linux-4.9.93-linuxkit-aufs-x86_64-with-debian-9.4 2018-10-29 17:25:27
-- * - **** ---
- ** ---------- [config]
- ** ---------- .> app: celeryworker:0x7f28e828f668
- ** ---------- .> transport: amqp://guest:**#rabbit1:5672//
- ** ---------- .> results: mongodb://
- *** --- * --- .> concurrency: 2 (prefork)
-- ******* ---- .> task events: OFF (enable -E to monitor tasks in this worker)
--- ***** -----
-------------- [queues]
.> celery exchange=celery(direct) key=celery
[tasks]
. celeryworker.tasks.longtime_add

Answer 1

I still do not know why the above config is not working but I found a workaround to update the config after the app loads using the new config value names:
app = Celery('celeryworker', broker=os.getenv('CELERY_BROKER_URL'),
backend=os.getenv('CELERY_RESULT_BACKEND'),
include=['SGSDevOps.celeryworker.tasks'])
print('app initiated')
app.config_from_object(app_settings)
app.conf.update(accept_content=['json'])
app.conf.update(mongodb_backend_settings=ast.literal_eval(os.getenv('CELERY_MONGODB_BACKEND_SETTINGS')))