fix: resolve MySQL Error 1093 when deleting users from boards

Fixes #7125 and #7069 by implementing a two-step deletion process
that avoids MySQL's restriction on deleting from a table while
selecting from it in a subquery.

The fix separates the SELECT and DELETE operations:
1. First query: Get card IDs for assignments to delete
2. Second query: Delete assignments using the collected IDs

This approach works on all supported database systems (MySQL 5.7+,
MySQL 8.0+, MariaDB 10.x+) and follows MySQL's official best practices
for handling Error 1093: 'You can't specify target table for update in FROM clause'.

The issue occurred because the original deleteByParticipantOnBoard method
used a subquery that referenced the same table being deleted from,
which MySQL prohibits but MariaDB allows (explaining why it worked
in development but failed in production).

Signed-off-by: Arne Bartelt <[email protected]>
Signed-off-by: Arne Bartelt <[email protected]>

Author: ABartelt

AUTHORS.md

@@ -6,6 +6,7 @@
 
 - Adrian Missy <[email protected]>
 - Alexandru Puiu <[email protected]>
+- Arne Bartelt <[email protected]>
 - Chandi Langecker <[email protected]>
 - Christoph Wurst <[email protected]>
 - Gary Kim <[email protected]>

lib/Db/AssignmentMapper.php

@@ -77,18 +77,33 @@ public function findByParticipant(string $participant, $type = Assignment::TYPE_
 	}
 
 	public function deleteByParticipantOnBoard(string $participant, int $boardId, $type = Assignment::TYPE_USER) {
-		$qb = $this->db->getQueryBuilder();
+		// Step 1: Get all card IDs for the board that have assignments for this participant
+		// This avoids MySQL Error 1093 by separating the SELECT from the DELETE operation
 		$cardIdQuery = $this->db->getQueryBuilder();
 		$cardIdQuery->select('a.card_id')
 			->from('deck_assigned_users', 'a')
 			->innerJoin('a', 'deck_cards', 'c', 'c.id = a.card_id')
 			->innerJoin('c', 'deck_stacks', 's', 's.id = c.stack_id')
-			->where($cardIdQuery->expr()->eq('a.participant', $qb->createNamedParameter($participant, IQueryBuilder::PARAM_STR)))
-			->andWhere($cardIdQuery->expr()->eq('s.board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
-			->andWhere($cardIdQuery->expr()->eq('a.type', $qb->createNamedParameter($type, IQueryBuilder::PARAM_INT)));
-		$qb->delete('deck_assigned_users')
-			->where($qb->expr()->in('card_id', $qb->createFunction($cardIdQuery->getSQL()), IQueryBuilder::PARAM_INT_ARRAY));
-		$qb->executeStatement();
+			->where($cardIdQuery->expr()->eq('a.participant', $cardIdQuery->createNamedParameter($participant, IQueryBuilder::PARAM_STR)))
+			->andWhere($cardIdQuery->expr()->eq('s.board_id', $cardIdQuery->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->andWhere($cardIdQuery->expr()->eq('a.type', $cardIdQuery->createNamedParameter($type, IQueryBuilder::PARAM_INT)));
+
+		$result = $cardIdQuery->executeQuery();
+		$cardIds = [];
+		while ($row = $result->fetch()) {
+			$cardIds[] = $row['card_id'];
+		}
+		$result->closeCursor();
+
+		// Step 2: If we have card IDs, delete the assignments
+		if (!empty($cardIds)) {
+			$deleteQuery = $this->db->getQueryBuilder();
+			$deleteQuery->delete('deck_assigned_users')
+				->where($deleteQuery->expr()->eq('participant', $deleteQuery->createNamedParameter($participant, IQueryBuilder::PARAM_STR)))
+				->andWhere($deleteQuery->expr()->eq('type', $deleteQuery->createNamedParameter($type, IQueryBuilder::PARAM_INT)))
+				->andWhere($deleteQuery->expr()->in('card_id', $deleteQuery->createNamedParameter($cardIds, IQueryBuilder::PARAM_INT_ARRAY)));
+			$deleteQuery->executeStatement();
+		}
 	}