Hi Bubblers,
Just released a plugin that lets you connect to MongoDB Atlas, paid version coming soon that avoids having to setup clusters and allows you to save data directly out of the box.
Service Link: https://mongodbapi.bubbleapps.io/version-test
Plugin Page - MongoDB Plugin | Bubble
Instructions
Authentication 
To get started, setup your Cloud MongoDB and create a cluster.
Create a Data API Key and enter your secret key in the Bubble.io plugin section of your web app. Your secret is only revealed once in your Atlas dashboard.
Mutate Data 
Ensure you have the name for your cluster, database and collection.
You will need your cluster name to begin adding data and changing data. Ensure you have also created a database and collection to reference where you will be storing your data.
In Bubble you will need to form objects using a JSON plugin or embed data directly into JSON to successfully query your database.
EXAMPLES COMING SOON