As one can imagine, a SQL ORM cannot be used with a NoSQL database. However, this is usually not a problem, because the ideal use cases for NoSQL databases typically do not require the functionality present in an ORM (namely, relation support).
With a NoSQL databases, you can use the Service API (you likely already are!), and use Service.map to deal with Dart data only, rather than messing around with Maps, and risking typos and refactoring challenges.
If you are using package:angel_serialize, this is pretty easy:
String get text;
double get attachedMoney;
var service =MongoService(...);
var mappedService = service.map(GreetingSerializer.fromMap, GreetingSerializer.toMap);