MongoEngine connects to incorrect database during testing - mongodb

Question

Context
I'm creating Flask app connected to mongodb using MongoEngine via flask-mongoengine extension. I create my app using application factory pattern as specified in configuration instructions.
Problem
While running test(s), I specified testing database named datazilla_test which is passed to mongo instance via mongo.init_app(app). Even though my app.config['MONGODB_DB'] and mongo.app.config['MONGODB_DB'] instance has correct value (datazilla_test), this value is not reflected in mongo instance. Thus, when I run assertion assert mongo.get_db().name == mongo.app.config['MONGODB_DB'] this error is triggered AssertionError: assert 'datazzilla' == 'datazzilla_test'
Question
What am I doing wrong? Why database connection persist with default database datazzilla rather, than datazilla_test? How to fix it?
Source Code
# __init__.py
from flask_mongoengine import MongoEngine
mongo = MongoEngine()
def create_app(config=None):
app = Flask(__name__)
app.config['MONGODB_HOST'] = 'localhost'
app.config['MONGODB_PORT'] = '27017'
app.config['MONGODB_DB'] = 'datazzilla'
# override default config
if config is not None:
app.config.from_mapping(config)
mongo.init_app(app)
return app
# conftest.py
import pytest
from app import mongo
from app import create_app
#pytest.fixture
def app():
app = create_app({
'MONGODB_DB': 'datazzilla_test',
})
assert mongo.get_db().name == mongo.app.config['MONGODB_DB']
# AssertionError: assert 'datazzilla' == 'datazzilla_test'
return app

Answer 1

Mongoengine is already connected when your fixture is called, when you call app = create_app from your fixture, it tries to re-establish the connection but fails silently (as it sees that there is an existing default connection established).
This got reworked in the development version of mongoengine (see https://github.com/MongoEngine/mongoengine/pull/2038) but wasn't released yet (As of 04-JUN-2019). When that version gets out, you'll be able to disconnect any existing mongoengine connection by calling disconnect_all
In the meantime, you can either:
- check where the existing connection is created and prevent it
- try to disconnect the existing connection by using the following:
from mongoengine.connection import disconnect, _connection_settings
#pytest.fixture
def app():
disconnect()
del _connection_settings['default']
app = create_app(...)
...
But it may have other side effects

Answer 2

Context
Coincidentally, I figure-out fix for this problem. #bagerard answer is correct! It works for MongoClient where client's connect is set to True -this is/should be default value.
MongoClient(host=['mongo:27017'], document_class=dict, tz_aware=False, connect=False, read_preference=Primary())
If that is the case, then you have to disconnect database and delete connection settings as #bagerard explains.
Solution
However, if you change MongoClient connection to False, then you don't have to disconnect database and delete connection settings. At the end solution that worked for me was this solution.
def create_app(config=None):
...
app.config['MONGODB_CONNECT'] = False
...
Notes
As I wrote earlier. I found this solution coincidentally, I was trying to solve this problem MongoClient opened before fork. Create MongoClient only after forking. It turned out that it fixes both problems :)
P.S If there are any side effects I'm not aware of them at this point! If you find some then please share them in comments section.