dms: explicitly cast `path` selected columns to char
On python3, the type of selected columns depend on the data type from
mariadb side, VARCHAR will be str, BINARY/BLOB will be bytes, etc
These SQL method select path that is first evaluated from a variable
that is NULL and in that case, mariadb seems to select LONGBLOB as data
type:
MariaDB [test]> set @defined_as_null=null; drop table if exists tmp; create table tmp as (select @defined_as_null); show create table tmp;
+-------+------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------------------------------------------------------+
| tmp | CREATE TABLE `tmp` (
`@defined_as_null` longblob DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci |
+-------+------------------------------------------------------------------------------------------------------------------------------------+
By casting to CHAR in SQL, on the python side we always have the str
that we expect here, because this is used as path attribute of a SQL
brain.
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