'Endpoint to retrieve values for specified column.
:param datasource_type: Type of datasource e.g. table
:param datasource_id: Datasource id
:param column: Column name to retrieve values for
:return:'

@api
@has_access_api
@expose(u'/filter/<datasource_type>/<datasource_id>/<column>/')
def filter(self, datasource_type, datasource_id, column):

    datasource_class = ConnectorRegistry.sources[datasource_type]
    datasource = db.session.query(datasource_class).filter_by(id=datasource_id).first()
    if (not datasource):
        return json_error_response(DATASOURCE_MISSING_ERR)
    if (not self.datasource_access(datasource)):
        return json_error_response(DATASOURCE_ACCESS_ERR)
    payload = json.dumps(datasource.values_for_column(column), default=utils.json_int_dttm_ser)
    return json_success(payload)
