Why does GenericReferenceField wrap dbref ?

Hi !

In order to migrate some data to GenericReferenceField, I simply used a DBRef as defined in MongoDB documentation: https://docs.mongodb.org/v3.0/reference/database-references/
It took me some time and some code reading to understand that GenericReferenceField wraps DBRef in a {_cls: 'ClassName', _ref: DBRef} object.

So, I'm wondering why is GenericReferenceField using this pattern instead of the standard DBRef. It's harder to query, harder to read and to me the _cls is useless because:

1. if the referenced document does not allow inheritance, there is only one class in the collection (which dbref provide)
2. if the referenced document use inheritance, the _cls attribute is stored in the document itself so the collection is sufficient too

At first I was thinking that it permit, in the case of inheritance, to only fetch the required attribute on dereferencing but, it's not allowed anymore to store more field that the model has and this reasoning does not stand for DynamicDocument.

So can someone explain me this choice? Is there some historical reason ? Am I missing something ?

Start with GenericReferenceField's handling of MongoDB DBRef and compare it with the MongoDB database-reference documentation linked in the issue. Determine whether the _cls/_ref wrapper is intentional and document the historical or inheritance-related rationale; the issue is resolved when that rationale is clearly explained.