GET /datastores/{datastore_id}/datasources/{datasource_id}/columns: Get a
            specific datasource's columns within a given datastore.

def showcolumns(self, datastore_id=None, datasource_id=None):
        """ GET /datastores/{datastore_id}/datasources/{datasource_id}/columns: Get a
            specific datasource's columns within a given datastore. """
        datastore = self._get_datastore_by_id(datastore_id)
        if datastore is None:
            abort(404)
        discovery_result = discover_datasource_columns(datastore.ogrstring, datasource_id)
        if discovery_result is None:
            abort(404)
        # prevent JSON Array Cross-site Exploits (XSRF/CSRF)
        return {"columns": discovery_result}