The url shortener microservice project was my first back-end project that required a database. I used MongoDB to take a URL, create an ID for that URL, and return both the original submitted URL and the new 'shortened' URL using the ID parameter.
One tricky part about this project was getting the database hosted somewhere and hooking up the Heroku app to that database. I used mLab for hosting my Mongo database free of charge!
The Server-side Development with NodeJS course on Coursera was instrumental for this project. I wouldn't have made it through without the guidance this course provided. It covers Node, Express, MongoDB, and Mongoose, so you can use it to learn everything you might need for the M, E, and N parts of the MEAN stack.