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

        ref = DBRef(id=ref, collection=self.type.type.get_collection_name(),
                    database=self.db)
        ref.type = self.type.type
        return session.dereference(ref, allow_none=allow_none)