InvalidDocument when using EmbeddedDocument primary key and ReferenceField

The mongoengine documentation states that EmbeddedDocuments support being the primary_key, which is true, but this breaks if you try to create a ReferenceField pointing to such a document. Consider the following:

import mongoengine as me
from bson import ObjectId

class IdTag(me.EmbeddedDocument):
    id = me.ObjectIdField(default=ObjectId)
    tag = StringField(default='')

class VersionedDocument(me.Document):
    id = me.EmbeddedDocumentField(IdTag, default=IdTag, primary_key=True)
    meta = {'abstract': True}

class A(VersionedDocument):
    name = me.StringField()

class B(VersionedDocument):
    name = me.StringField()
    a = me.ReferenceField(A)

> a1 = A(name="a1").save()
> b1 = B(name="b1", a=a1).save()
> B.objects.get().a

This raises an InvalidDocument exception during dereferencing. Pymongo uses the id of the dbref directly in a query:

return self[dbref.collection].find_one({"_id": dbref.id}, session=session, **kwargs)

The relevant call in mongoengine is Line 1178 in fields.py

dereferenced_son = ref_cls._get_db().dereference(dbref)

I think this could be universally fixed with

dereferenced_son = ref_cls._get_db().dereference(DBRef(dbref.collection, dbref.id.to_mongo())

Unless dbref.id is not always a field, in which case a try..except should work

Start in mongoengine/fields.py at line 1178 and reproduce the InvalidDocument with the EmbeddedDocument primary key and ReferenceField example. Compare the dbref handling there with PyMongo's dereference call. Done means B.objects.get().a dereferences successfully without InvalidDocument, with coverage for the reported case.