diff --git a/lib/Db/AclMapper.php b/lib/Db/AclMapper.php
index 5cf188aa0..f3d6ea3d9 100644
--- a/lib/Db/AclMapper.php
+++ b/lib/Db/AclMapper.php
@@ -33,18 +33,45 @@ class AclMapper extends DeckMapper implements IPermissionMapper {
 		parent::__construct($db, 'deck_board_acl', Acl::class);
 	}
 
+	/**
+	 * @param numeric $boardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Acl[]
+	 * @throws \OCP\DB\Exception
+	 */
 	public function findAll($boardId, $limit = null, $offset = null) {
-		$sql = 'SELECT id, board_id, type, participant, permission_edit, permission_share, permission_manage FROM `*PREFIX*deck_board_acl` WHERE `board_id` = ? ';
-		return $this->findEntities($sql, [$boardId], $limit, $offset);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('id', 'board_id', 'type', 'participant', 'permission_edit', 'permission_share', 'permission_manage')
+			->from('deck_board_acl')
+			->where($qb->expr()->eq('board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->setMaxResults($limit)
+			->setFirstResult($offset);
+
+		return $this->findEntities($qb);
 	}
 
+	/**
+	 * @param numeric $userId
+	 * @param numeric $aclId
+	 * @return bool
+	 * @throws \OCP\DB\Exception
+	 */
 	public function isOwner($userId, $aclId): bool {
-		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_board_acl` WHERE id = ?)';
-		$stmt = $this->execute($sql, [$aclId]);
-		$row = $stmt->fetch();
-		return ($row['owner'] === $userId);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('acl.id')
+			->from($this->getTableName(), 'acl')
+			->innerJoin('acl', 'deck_boards', 'b', 'acl.board_id = b.id')
+			->where($qb->expr()->eq('owner', $qb->createNamedParameter($userId, IQueryBuilder::PARAM_STR)))
+			->andWhere($qb->expr()->eq('acl.id', $qb->createNamedParameter($aclId, IQueryBuilder::PARAM_INT)));
+
+		return count($qb->executeQuery()->fetchAll()) > 0;
 	}
 
+	/**
+	 * @param numeric $id
+	 * @return int|null
+	 */
 	public function findBoardId($id): ?int {
 		try {
 			$entity = $this->find($id);
@@ -54,9 +81,21 @@ class AclMapper extends DeckMapper implements IPermissionMapper {
 		return null;
 	}
 
+	/**
+	 * @param int $type
+	 * @param string $participant
+	 * @return Acl[]
+	 * @throws \OCP\DB\Exception
+	 */
 	public function findByParticipant($type, $participant): array {
-		$sql = 'SELECT * from *PREFIX*deck_board_acl WHERE type = ? AND participant = ?';
-		return $this->findEntities($sql, [$type, $participant]);
+		$qb = $this->db->getQueryBuilder();
+
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('type', $qb->createNamedParameter($type, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->eq('participant', $qb->createNamedParameter($participant, IQueryBuilder::PARAM_STR)));
+
+		return $this->findEntities($qb);
 	}
 
 	/**
diff --git a/lib/Db/AssignmentMapper.php b/lib/Db/AssignmentMapper.php
index fdbf80d85..98a2ead71 100644
--- a/lib/Db/AssignmentMapper.php
+++ b/lib/Db/AssignmentMapper.php
@@ -55,9 +55,6 @@ class AssignmentMapper extends QBMapper implements IPermissionMapper {
 		$this->circleService = $circleService;
 	}
 
-	/**
-	 * @return Assignment[]
-	 */
 	public function findAll(int $cardId): array {
 		$qb = $this->db->getQueryBuilder();
 		$qb->select('*')
diff --git a/lib/Db/AttachmentMapper.php b/lib/Db/AttachmentMapper.php
index 46e63f7c4..727e3cce1 100644
--- a/lib/Db/AttachmentMapper.php
+++ b/lib/Db/AttachmentMapper.php
@@ -30,7 +30,6 @@ use OCP\AppFramework\Db\MultipleObjectsReturnedException;
 use OCP\DB\QueryBuilder\IQueryBuilder;
 use OCP\IDBConnection;
 use OCP\IUserManager;
-use PDO;
 
 class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 	private $cardMapper;
@@ -52,70 +51,53 @@ class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 	}
 
 	/**
-	 * @param $id
-	 * @return Entity|Attachment
-	 * @throws \OCP\AppFramework\Db\DoesNotExistException
-	 * @throws \OCP\AppFramework\Db\MultipleObjectsReturnedException
+	 * @param int $id
+	 * @return Attachment
+	 * @throws DoesNotExistException
+	 * @throws MultipleObjectsReturnedException
+	 * @throws \OCP\DB\Exception
 	 */
 	public function find($id) {
 		$qb = $this->db->getQueryBuilder();
 		$qb->select('*')
-			->from('deck_attachment')
+			->from($this->getTableName())
 			->where($qb->expr()->eq('id', $qb->createNamedParameter($id, IQueryBuilder::PARAM_INT)));
 
-		$cursor = $qb->execute();
-		$row = $cursor->fetch(PDO::FETCH_ASSOC);
-		if ($row === false) {
-			$cursor->closeCursor();
-			throw new DoesNotExistException('Did expect one result but found none when executing' . $qb);
-		}
-
-		$row2 = $cursor->fetch();
-		$cursor->closeCursor();
-		if ($row2 !== false) {
-			throw new MultipleObjectsReturnedException('Did not expect more than one result when executing' . $query);
-		}
-
-		return $this->mapRowToEntity($row);
-	}
-
-	public function findByData($cardId, $data) {
-		$qb = $this->db->getQueryBuilder();
-		$qb->select('*')
-				->from('deck_attachment')
-				->where($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)))
-				->andWhere($qb->expr()->eq('data', $qb->createNamedParameter($data, IQueryBuilder::PARAM_STR)));
-		$cursor = $qb->execute();
-		$row = $cursor->fetch(PDO::FETCH_ASSOC);
-		if ($row === false) {
-			$cursor->closeCursor();
-			throw new DoesNotExistException('Did expect one result but found none when executing' . $qb);
-		}
-		$cursor->closeCursor();
-		return $this->mapRowToEntity($row);
+		return $this->findEntity($qb);
+	}
+
+	/**
+	 * @param int $cardId
+	 * @param string $data
+	 * @return Attachment
+	 * @throws DoesNotExistException
+	 * @throws MultipleObjectsReturnedException
+	 * @throws \OCP\DB\Exception
+	 */
+	public function findByData($cardId, $data) {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+				->from($this->getTableName())
+				->where($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)))
+				->andWhere($qb->expr()->eq('data', $qb->createNamedParameter($data, IQueryBuilder::PARAM_STR)));
+
+		return $this->findEntity($qb);
 	}
 
 	/**
-	 * Find all attachments for a card
-	 *
 	 * @param $cardId
-	 * @return array
+	 * @return Entity[]
+	 * @throws \OCP\DB\Exception
 	 */
 	public function findAll($cardId) {
 		$qb = $this->db->getQueryBuilder();
 		$qb->select('*')
-			->from('deck_attachment')
+			->from($this->getTableName())
 			->where($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)))
 			->andWhere($qb->expr()->eq('deleted_at', $qb->createNamedParameter(0, IQueryBuilder::PARAM_INT)));
 
 
-		$entities = [];
-		$cursor = $qb->execute();
-		while ($row = $cursor->fetch()) {
-			$entities[] = $this->mapRowToEntity($row);
-		}
-		$cursor->closeCursor();
-		return $entities;
+		return $this->findEntities($qb);
 	}
 
 	/**
@@ -128,7 +110,7 @@ class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 		$timeLimit = time() - (60 * 5);
 		$qb = $this->db->getQueryBuilder();
 		$qb->select('*')
-			->from('deck_attachment')
+			->from($this->getTableName())
 			->where($qb->expr()->gt('deleted_at', $qb->createNamedParameter(0, IQueryBuilder::PARAM_INT)));
 		if ($withOffset) {
 			$qb
@@ -139,13 +121,7 @@ class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 				->andWhere($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)));
 		}
 
-		$entities = [];
-		$cursor = $qb->execute();
-		while ($row = $cursor->fetch()) {
-			$entities[] = $this->mapRowToEntity($row);
-		}
-		$cursor->closeCursor();
-		return $entities;
+		return $this->findEntities($qb);
 	}
 
 
diff --git a/lib/Db/DeckMapper.php b/lib/Db/DeckMapper.php
index 2dd0dd990..0aa0e7d96 100644
--- a/lib/Db/DeckMapper.php
+++ b/lib/Db/DeckMapper.php
@@ -23,17 +23,15 @@
 
 namespace OCA\Deck\Db;
 
-use OCP\AppFramework\Db\Mapper;
+use OCP\AppFramework\Db\QBMapper;
+use OCP\DB\QueryBuilder\IQueryBuilder;
 
 /**
  * Class DeckMapper
  *
  * @package OCA\Deck\Db
- * @deprecated use QBMapper
- *
- * TODO: Move to QBMapper once Nextcloud 14 is a minimum requirement
  */
-class DeckMapper extends Mapper {
+class DeckMapper extends QBMapper {
 
 	/**
 	 * @param $id
@@ -42,11 +40,11 @@ class DeckMapper extends Mapper {
 	 * @throws \OCP\AppFramework\Db\DoesNotExistException
 	 */
 	public function find($id) {
-		$sql = 'SELECT * FROM `' . $this->tableName . '` ' . 'WHERE `id` = ?';
-		return $this->findEntity($sql, [$id]);
-	}
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('id', $qb->createNamedParameter($id, IQueryBuilder::PARAM_INT)));
 
-	protected function execute($sql, array $params = [], $limit = null, $offset = null) {
-		return parent::execute($sql, $params, $limit, $offset);
+		return $this->findEntity($qb);
 	}
 }
diff --git a/lib/Db/LabelMapper.php b/lib/Db/LabelMapper.php
index cc0b9b553..61ce03ed3 100644
--- a/lib/Db/LabelMapper.php
+++ b/lib/Db/LabelMapper.php
@@ -26,6 +26,7 @@ namespace OCA\Deck\Db;
 use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\Entity;
 use OCP\AppFramework\Db\MultipleObjectsReturnedException;
+use OCP\DB\QueryBuilder\IQueryBuilder;
 use OCP\IDBConnection;
 
 class LabelMapper extends DeckMapper implements IPermissionMapper {
@@ -33,41 +34,105 @@ class LabelMapper extends DeckMapper implements IPermissionMapper {
 		parent::__construct($db, 'deck_labels', Label::class);
 	}
 
-	public function findAll($boardId, $limit = null, $offset = null) {
-		$sql = 'SELECT * FROM `*PREFIX*deck_labels` WHERE `board_id` = ? ORDER BY `id`';
-		return $this->findEntities($sql, [$boardId], $limit, $offset);
+	/**
+	 * @param numeric $boardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Label[]
+	 * @throws \OCP\DB\Exception
+	 */
+	public function findAll($boardId, $limit = null, $offset = null): array {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->setMaxResults($limit)
+			->setFirstResult($offset);
+		return $this->findEntities($qb);
 	}
 
-	public function delete(\OCP\AppFramework\Db\Entity $entity) {
+	/**
+	 * @param Entity $entity
+	 * @return Entity
+	 * @throws \OCP\DB\Exception
+	 */
+	public function delete(Entity $entity): Entity {
 		// delete assigned labels
 		$this->deleteLabelAssignments($entity->getId());
 		// delete label
 		return parent::delete($entity);
 	}
 
-	public function findAssignedLabelsForCard($cardId, $limit = null, $offset = null) {
-		$sql = 'SELECT l.*,card_id FROM `*PREFIX*deck_assigned_labels` as al INNER JOIN *PREFIX*deck_labels as l ON l.id = al.label_id WHERE `card_id` = ? ORDER BY l.id';
-		return $this->findEntities($sql, [$cardId], $limit, $offset);
-	}
-	public function findAssignedLabelsForBoard($boardId, $limit = null, $offset = null) {
-		$sql = 'SELECT c.id as card_id, l.id as id, l.title as title, l.color as color FROM `*PREFIX*deck_cards` as c ' .
-			' INNER JOIN `*PREFIX*deck_assigned_labels` as al ON al.card_id = c.id INNER JOIN `*PREFIX*deck_labels` as l ON  al.label_id = l.id WHERE board_id=? ORDER BY l.id';
-		return $this->findEntities($sql, [$boardId], $limit, $offset);
+	/**
+	 * @param numeric $cardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Label[]
+	 * @throws \OCP\DB\Exception
+	 */
+	public function findAssignedLabelsForCard($cardId, $limit = null, $offset = null): array {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('l.*', 'card_id')
+			->from($this->getTableName(), 'l')
+			->innerJoin('l', 'deck_assigned_labels', 'al', 'l.id = al.label_id')
+			->where($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)))
+			->orderBy('l.id')
+			->setMaxResults($limit)
+			->setFirstResult($offset);
+
+		return $this->findEntities($qb);
 	}
 
-	public function insert(Entity $entity) {
+	/**
+	 * @param numeric $boardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Label[]
+	 * @throws \OCP\DB\Exception
+	 */
+	public function findAssignedLabelsForBoard($boardId, $limit = null, $offset = null): array {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('l.id as id', 'l.title as title', 'l.color as color')
+			->selectAlias('c.id', 'card_id')
+			->from($this->getTableName(), 'l')
+			->innerJoin('l', 'deck_assigned_labels', 'al', 'al.label_id = l.id')
+			->innerJoin('l', 'deck_cards', 'c', 'al.card_id = c.id')
+			->where($qb->expr()->eq('board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->orderBy('l.id')
+			->setMaxResults($limit)
+			->setFirstResult($offset);
+
+		return $this->findEntities($qb);
+	}
+
+	/**
+	 * @param Entity $entity
+	 * @return Entity
+	 * @throws \OCP\DB\Exception
+	 */
+	public function insert(Entity $entity): Entity {
 		$entity->setLastModified(time());
 		return parent::insert($entity);
 	}
 
-	public function update(Entity $entity, $updateModified = true) {
+	/**
+	 * @param Entity $entity
+	 * @param bool $updateModified
+	 * @return Entity
+	 * @throws \OCP\DB\Exception
+	 */
+	public function update(Entity $entity, $updateModified = true): Entity {
 		if ($updateModified) {
 			$entity->setLastModified(time());
 		}
 		return parent::update($entity);
 	}
 
-
+	/**
+	 * @param numeric $boardId
+	 * @return array
+	 * @throws \OCP\DB\Exception
+	 */
 	public function getAssignedLabelsForBoard($boardId) {
 		$labels = $this->findAssignedLabelsForBoard($boardId);
 		$result = [];
@@ -80,27 +145,51 @@ class LabelMapper extends DeckMapper implements IPermissionMapper {
 		return $result;
 	}
 
+	/**
+	 * @param numeric $labelId
+	 * @return void
+	 * @throws \OCP\DB\Exception
+	 */
 	public function deleteLabelAssignments($labelId) {
-		$sql = 'DELETE FROM `*PREFIX*deck_assigned_labels` WHERE label_id = ?';
-		$stmt = $this->db->prepare($sql);
-		$stmt->bindParam(1, $labelId, \PDO::PARAM_INT, 0);
-		$stmt->execute();
+		$qb = $this->db->getQueryBuilder();
+		$qb->delete('deck_assigned_labels')
+			->where($qb->expr()->eq('label_id', $qb->createNamedParameter($labelId, IQueryBuilder::PARAM_INT)));
+		$qb->executeStatement();
 	}
 
+	/**
+	 * @param numeric $cardId
+	 * @return void
+	 * @throws \OCP\DB\Exception
+	 */
 	public function deleteLabelAssignmentsForCard($cardId) {
-		$sql = 'DELETE FROM `*PREFIX*deck_assigned_labels` WHERE card_id = ?';
-		$stmt = $this->db->prepare($sql);
-		$stmt->bindParam(1, $cardId, \PDO::PARAM_INT, 0);
-		$stmt->execute();
+		$qb = $this->db->getQueryBuilder();
+		$qb->delete('deck_assigned_labels')
+			->where($qb->expr()->eq('card_id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)));
+		$qb->executeStatement();
 	}
 
+	/**
+	 * @param string $userId
+	 * @param numeric $labelId
+	 * @return bool
+	 * @throws \OCP\DB\Exception
+	 */
 	public function isOwner($userId, $labelId): bool {
-		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_labels` WHERE id = ?)';
-		$stmt = $this->execute($sql, [$labelId]);
-		$row = $stmt->fetch();
-		return ($row['owner'] === $userId);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('l.id')
+			->from($this->getTableName(), 'l')
+			->innerJoin('l', 'deck_boards', 'b', 'l.board_id = b.id')
+			->where($qb->expr()->eq('l.id', $qb->createNamedParameter($labelId, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->eq('b.owner', $qb->createNamedParameter($userId, IQueryBuilder::PARAM_STR)));
+
+		return count($qb->executeQuery()->fetchAll()) > 0;
 	}
 
+	/**
+	 * @param numeric $id
+	 * @return int|null
+	 */
 	public function findBoardId($id): ?int {
 		try {
 			$entity = $this->find($id);
diff --git a/lib/Db/StackMapper.php b/lib/Db/StackMapper.php
index 300cfd335..1ae59dd1b 100644
--- a/lib/Db/StackMapper.php
+++ b/lib/Db/StackMapper.php
@@ -26,6 +26,7 @@ namespace OCA\Deck\Db;
 use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\Entity;
 use OCP\AppFramework\Db\MultipleObjectsReturnedException;
+use OCP\DB\QueryBuilder\IQueryBuilder;
 use OCP\IDBConnection;
 
 class StackMapper extends DeckMapper implements IPermissionMapper {
@@ -38,62 +39,112 @@ class StackMapper extends DeckMapper implements IPermissionMapper {
 
 
 	/**
-	 * @param $id
-	 * @throws MultipleObjectsReturnedException
+	 * @param numeric $id
+	 * @return Stack
 	 * @throws DoesNotExistException
+	 * @throws MultipleObjectsReturnedException
+	 * @throws \OCP\DB\Exception
 	 */
 	public function find($id): Stack {
-		$sql = 'SELECT * FROM `*PREFIX*deck_stacks` ' .
-			'WHERE `id` = ?';
-		return $this->findEntity($sql, [$id]);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('id', $qb->createNamedParameter($id, IQueryBuilder::PARAM_INT)));
+
+		return $this->findEntity($qb);
 	}
 
 	/**
 	 * @param $cardId
 	 * @return Stack|null
+	 * @throws \OCP\DB\Exception
 	 */
 	public function findStackFromCardId($cardId): ?Stack {
-		$sql = <<<SQL
-SELECT s.* 
-FROM `*PREFIX*deck_stacks` as `s`
-INNER JOIN `*PREFIX*deck_cards` as `c` ON s.id = c.stack_id
-WHERE c.id = ?
-SQL;
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName(), 's')
+			->innerJoin('s', 'deck_cards', 'c', 's.id = c.stack_id')
+			->where($qb->expr()->eq('c.id', $qb->createNamedParameter($cardId, IQueryBuilder::PARAM_INT)));
+
 		try {
-			return $this->findEntity($sql, [$cardId]);
+			return $this->findEntity($qb);
 		} catch (MultipleObjectsReturnedException|DoesNotExistException $e) {
 		}
 
 		return null;
 	}
 
+	/**
+	 * @param numeric $boardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Stack[]
+	 * @throws \OCP\DB\Exception
+	 */
+	public function findAll($boardId, $limit = null, $offset = null): array {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->eq('deleted_at', $qb->createNamedParameter(0, IQueryBuilder::PARAM_INT)))
+			->setFirstResult($offset)
+			->setMaxResults($limit);
 
-	public function findAll($boardId, $limit = null, $offset = null) {
-		$sql = 'SELECT * FROM `*PREFIX*deck_stacks` WHERE `board_id` = ? AND deleted_at = 0 ORDER BY `order`, `id`';
-		return $this->findEntities($sql, [$boardId], $limit, $offset);
+		return $this->findEntities($qb);
 	}
 
-
+	/**
+	 * @param numeric $boardId
+	 * @param int|null $limit
+	 * @param int|null $offset
+	 * @return Stack[]
+	 * @throws \OCP\DB\Exception
+	 */
 	public function findDeleted($boardId, $limit = null, $offset = null) {
-		$sql = 'SELECT * FROM `*PREFIX*deck_stacks` s
-	  WHERE `s`.`board_id` = ? AND NOT s.deleted_at = 0';
-		return $this->findEntities($sql, [$boardId], $limit, $offset);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')
+			->from($this->getTableName())
+			->where($qb->expr()->eq('board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->neq('deleted_at', $qb->createNamedParameter(0, IQueryBuilder::PARAM_INT)))
+			->setFirstResult($offset)
+			->setMaxResults($limit);
+
+		return $this->findEntities($qb);
 	}
 
-
-	public function delete(Entity $entity) {
+	/**
+	 * @param Entity $entity
+	 * @return Entity
+	 * @throws \OCP\DB\Exception
+	 */
+	public function delete(Entity $entity): Entity {
 		// delete cards on stack
 		$this->cardMapper->deleteByStack($entity->getId());
 		return parent::delete($entity);
 	}
 
+	/**
+	 * @param numeric $userId
+	 * @param numeric $stackId
+	 * @return bool
+	 * @throws \OCP\DB\Exception
+	 */
 	public function isOwner($userId, $stackId): bool {
-		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_stacks` WHERE id = ?)';
-		$stmt = $this->execute($sql, [$stackId]);
-		$row = $stmt->fetch();
-		return ($row['owner'] === $userId);
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('s.id')
+			->from($this->getTableName(), 's')
+			->innerJoin('s', 'deck_boards', 'b', 'b.id = s.board_id')
+			->where($qb->expr()->eq('s.id', $qb->createNamedParameter($stackId, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->eq('owner', $qb->createNamedParameter($userId, IQueryBuilder::PARAM_STR)));
+
+		return count($qb->executeQuery()->fetchAll()) > 0;
 	}
 
+	/**
+	 * @param numeric $id
+	 * @return int|null
+	 * @throws \OCP\DB\Exception
+	 */
 	public function findBoardId($id): ?int {
 		try {
 			$entity = $this->find($id);
diff --git a/lib/Service/BoardService.php b/lib/Service/BoardService.php
index e1cfc8023..4dd2b05c3 100644
--- a/lib/Service/BoardService.php
+++ b/lib/Service/BoardService.php
@@ -625,11 +625,9 @@ class BoardService {
 			} catch (\Exception $e) {
 			}
 		}
-		$delete = $this->aclMapper->delete($acl);
 
 		$this->eventDispatcher->dispatchTyped(new AclDeletedEvent($acl));
-
-		return $delete;
+		return (bool) $this->aclMapper->delete($acl);
 	}
 
 	/**
diff --git a/lib/Service/LabelService.php b/lib/Service/LabelService.php
index b27384760..f4c95fb66 100644
--- a/lib/Service/LabelService.php
+++ b/lib/Service/LabelService.php
@@ -91,12 +91,10 @@ class LabelService {
 		$this->permissionService->checkPermission(null, $boardId, Acl::PERMISSION_MANAGE);
 
 		$boardLabels = $this->labelMapper->findAll($boardId);
-		if (\is_array($boardLabels)) {
-			foreach ($boardLabels as $boardLabel) {
-				if ($boardLabel->getTitle() === $title) {
-					throw new BadRequestException('title must be unique');
-					break;
-				}
+		foreach ($boardLabels as $boardLabel) {
+			if ($boardLabel->getTitle() === $title) {
+				throw new BadRequestException('title must be unique');
+				break;
 			}
 		}
 
@@ -163,15 +161,13 @@ class LabelService {
 		$label = $this->find($id);
 
 		$boardLabels = $this->labelMapper->findAll($label->getBoardId());
-		if (\is_array($boardLabels)) {
-			foreach ($boardLabels as $boardLabel) {
-				if ($boardLabel->getId() === $label->getId()) {
-					continue;
-				}
-				if ($boardLabel->getTitle() === $title) {
-					throw new BadRequestException('title must be unique');
-					break;
-				}
+		foreach ($boardLabels as $boardLabel) {
+			if ($boardLabel->getId() === $label->getId()) {
+				continue;
+			}
+			if ($boardLabel->getTitle() === $title) {
+				throw new BadRequestException('title must be unique');
+				break;
 			}
 		}
 
diff --git a/tests/unit/Service/BoardServiceTest.php b/tests/unit/Service/BoardServiceTest.php
index 0e8bfeb15..702e16cd4 100644
--- a/tests/unit/Service/BoardServiceTest.php
+++ b/tests/unit/Service/BoardServiceTest.php
@@ -420,7 +420,7 @@ class BoardServiceTest extends TestCase {
 		$this->aclMapper->expects($this->once())
 			->method('delete')
 			->with($acl)
-			->willReturn(true);
+			->willReturn($acl);
 		$this->assertTrue($this->service->deleteAcl(123));
 	}
 }
diff --git a/tests/unit/Service/CardServiceTest.php b/tests/unit/Service/CardServiceTest.php
index 622b22c5b..ca260dc9b 100644
--- a/tests/unit/Service/CardServiceTest.php
+++ b/tests/unit/Service/CardServiceTest.php
@@ -160,6 +160,7 @@ class CardServiceTest extends TestCase {
 		$cardExpected->setAssignedUsers(['user1', 'user2']);
 		$cardExpected->setRelatedBoard($boardMock);
 		$cardExpected->setRelatedStack($stackMock);
+		$cardExpected->setLabels([]);
 		$this->assertEquals($cardExpected, $this->cardService->find(123));
 	}