Problem dereferencing non-binary UUID primary keys

Hi all,
I have encountered a problem dereferencing instances when that have a primary key that is a UUID that is stored in mongodb as a string.

class Pool(db.Document):
    uuid = db.UUIDField(required=True, primary_key=True, binary=False)

class Host(db.Document):
    uuid = db.UUIDField(required=True, primary_key=True, binary=False )
    pool = db.ReferenceField(Pool)

When I try to access to pool attribute I get a DBRef instance instead a Pool instance.

p = Pool(uuid="36a36f7b-54b7-724f-d28d-2262a629c18a")
h = Host(uuid="70a36f7b-1db7-724f-d28d-2262a629c36b")
h.pool = p
h.save()

host = Host.objects.get(uuid="70a36f7b-1db7-724f-d28d-2262a629c36b")
host.pool
DBRef('pool', UUID('36a36f7b-54b7-724f-d28d-2262a629c18a'))

The DBRef contains the UUID object but the value is stored as string in mongodb. So when this is used in the get of ReferenceField class the search is done using the UUID object instead of the string:

if self._auto_dereference and isinstance(value, DBRef):
    value = self.document_type._get_db().dereference(value)

end return None

If the uuid is stored using binary=True all works fine.

Start with ReferenceField.get, where the issue shows DBRef dereferencing, and trace how UUIDField values with binary=False are stored and looked up. Reproduce the example with string-backed UUID primary keys and compare it with binary=True. Done means accessing host.pool returns the Pool instance for non-binary UUID keys rather than a DBRef.