def asDictionary(self):
        """ returns the data source as a dictionary """
        return {
            "type": "joinTable",
            "leftTableSource": self._leftTableSource,
            "rightTableSource": self._rightTableSource,
            "leftTableKey": self._leftTableKey,
            "rightTableKey": self._rightTableKey,
            "joinType": self._joinType
        }
returns the data source as a dictionary