pip install flask-mongoengine

from flask import Flask
from flask.ext.mongoengine import MongoEngine
 
app = Flask(__name__)
app.config.from_pyfile('the-config.cfg')
db = MongoEngine(app)

Or, if you are setting up your database before your app is initialized….

from flask import Flask
from flask.ext.mongoengine import MongoEngine
db = MongoEngine()
...
app = Flask(__name__)
app.config.from_pyfile('the-config.cfg')
db.init_app(app)