[DandyDev]

We use MongoDB in conjunction with RealmDB to build an offline first mobile app. For this specifically it works very well. You basically define which parts of your document collection should be synced to which device, for example based on a query that contains the user id. MongoDB takes care of syncing the right data to the right device when internet connection is available. On the device, the data is stored locally in RealmDB, which represents the other side of the sync.

This is not easy to do with PostgreSQL, which we use for all other scenarios requiring a DB.