docs: def validate_unwrap(self, value, session=None):
          ''' Validates that the DBRef is valid as well as can be done without
              retrieving it.
          '''
          if not isinstance(value, DBRef):
              self._fail_validation_type(value, DBRef)
          if self.type:
              expected = self.type.type.get_collection_name()
              got = value.collection
              if expected != got:
                  self._fail_validation(value, '''Wrong collection for reference: '''
                                        '''got "%s" instead of "%s" ''' % (got, expected))
          if self.db_required and not value.database:
              self._fail_validation(value, 'db_required=True, but not database specified')
          if self.db and value.database and self.db != value.database:
              self._fail_validation(value, '''Wrong database for reference: '''
                                    ''' got "%s" instead of "%s" ''' % (value.database, self.db) )