You can use any of these three libraries

• Flask-PyMongo - https://flask-pymongo.readthedocs.io/en/latest/
• Flask-MongoAlchemy - https://pythonhosted.org/Flask-MongoAlchemy/
• Flask-MongoEngine - http://docs.mongoengine.org/projects/flask-mongoengine/en/latest/

I personally use flask mongoengine and every things work fine

I personally find the PyMongo library simple and easy to use.

You first need to import MongoClient and create a connection:

from pymongo import MongoClient
client = MongoClient()

Then get your db instance and collection (table):

db = client.my_database
collection = db.my_collection

You can then manipulate your data by working with JSON documents that hold your data. There is a complete example on their website.

Take a look at this tutorial on how to use PyMongo.

Maybe you don't have to use special libraries for Flask to connect to MongoDB.

Just treat Flask application as a normal Python program, and connect to MongoDB via normal Python libraries like PyMongo.

Here's an example by someone else. A Flask app connecting to MongoDB via PyMongo.