mongo.bson.iterator.type() and
mongo.bson.iterator.next() will return this constant (12L) to
indicate that the value pointed to by an iterator is a dbref (database
reference).
mongo.bson.dbrefint 12
mongo.bson.to.list() or
mongo.bson.iterator.value() will throw an error. The field
must be skipped by calling mongo.bson.iterator.next().
mongo.bson.iterator.type,
mongo.bson.iterator.next, mongo.bson.