It’s probably not correct to call Aura.Marshal an object-relational manager. With ORMs proper like Doctrine and Propel, the ORM issues queries for you using an embedded or preferred database library.
With Aura.Marshal, you use the data retrieval tools of your choice and write your own queries to retrieve data from a data source. You then load the result data into an entity type object, and the marshal automatically wires up entity and collection objects for you based on a relationship scheme you define for it.
The great benefit of this approach is that it completely decouples the in-memory object wiring from the act of doing the queries in the first place. The Marshal knows how to build and interrelate the objects, but it has no idea how to get the data for those objects in the first place.