FerretDB
A truly Open Source MongoDB alternative
...FerretDB is going to be a perfect solution for those looking for MongoDB development experience available as fully Open Source Software. The core of our solution is a stateless proxy, which converts MongoDB protocol queries to SQL, and uses PostgreSQL as a database engine. This will be compatible with MongoDB drivers, and should work as a drop-in replacement to MongoDB in many cases. Our vision is to enable the open source community and developers to reap the benefits of easy-to-use document databases, while staying away from vendor lock-in, and fauxpen licenses. ...