<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dereference(self, session, ref, allow_none=False):
 """ Dereference a pymongo "DBRef" to this field's underlying type """

        from ommongo.document import collection_registry
        # TODO: namespace support
        ref.type = collection_registry['global'][ref.collection]
        obj = session.dereference(ref, allow_none=allow_none)
        return obj