Extend support around the Extended Reference Pattern via DocumentReferences.

Explore if it makes sense to have a different extraction pattern for storing the relation than for the retrieval. Doing so would allow users to build dedicated views on the data for reading operations that require less data and must not resolve the reference.

class Payment {

    String id;

    @DocumentReference(fields = { "id", "name", "birthdate" }, lookup="{ '_id' : ?#{#target.id} }")
    Customer sender;

    PaymentReceiver receiver;

    // ...
}

Start by reviewing the existing DocumentReference support and the Extended Reference Pattern described in the issue. Compare how relation data is stored with how it is retrieved, including the Payment and Customer example. Done would require a decided design for separate extraction and retrieval patterns, along with agreed implementation and test scope.