nextcloud / server

lib/private/Comments/Manager.php

Indentation +1265 added lines, -1265 removed lines

@@ -46,1269 +46,1269 @@
 
 class Manager implements ICommentsManager {
 
-	/** @var  IDBConnection */
-	protected $dbConn;
-
-	/** @var  LoggerInterface */
-	protected $logger;
-
-	/** @var IConfig */
-	protected $config;
-
-	/** @var ITimeFactory */
-	protected $timeFactory;
-
-	/** @var IInitialStateService */
-	protected $initialStateService;
-
-	/** @var IComment[] */
-	protected $commentsCache = [];
-
-	/** @var  \Closure[] */
-	protected $eventHandlerClosures = [];
-
-	/** @var  ICommentsEventHandler[] */
-	protected $eventHandlers = [];
-
-	/** @var \Closure[] */
-	protected $displayNameResolvers = [];
-
-	public function __construct(IDBConnection $dbConn,
-								LoggerInterface $logger,
-								IConfig $config,
-								ITimeFactory $timeFactory,
-								IInitialStateService $initialStateService) {
-		$this->dbConn = $dbConn;
-		$this->logger = $logger;
-		$this->config = $config;
-		$this->timeFactory = $timeFactory;
-		$this->initialStateService = $initialStateService;
-	}
-
-	/**
-	 * converts data base data into PHP native, proper types as defined by
-	 * IComment interface.
-	 *
-	 * @param array $data
-	 * @return array
-	 */
-	protected function normalizeDatabaseData(array $data) {
-		$data['id'] = (string)$data['id'];
-		$data['parent_id'] = (string)$data['parent_id'];
-		$data['topmost_parent_id'] = (string)$data['topmost_parent_id'];
-		$data['creation_timestamp'] = new \DateTime($data['creation_timestamp']);
-		if (!is_null($data['latest_child_timestamp'])) {
-			$data['latest_child_timestamp'] = new \DateTime($data['latest_child_timestamp']);
-		}
-		$data['children_count'] = (int)$data['children_count'];
-		$data['reference_id'] = $data['reference_id'] ?? null;
-		return $data;
-	}
-
-
-	/**
-	 * @param array $data
-	 * @return IComment
-	 */
-	public function getCommentFromData(array $data): IComment {
-		return new Comment($this->normalizeDatabaseData($data));
-	}
-
-	/**
-	 * prepares a comment for an insert or update operation after making sure
-	 * all necessary fields have a value assigned.
-	 *
-	 * @param IComment $comment
-	 * @return IComment returns the same updated IComment instance as provided
-	 *                  by parameter for convenience
-	 * @throws \UnexpectedValueException
-	 */
-	protected function prepareCommentForDatabaseWrite(IComment $comment) {
-		if (!$comment->getActorType()
-			|| $comment->getActorId() === ''
-			|| !$comment->getObjectType()
-			|| $comment->getObjectId() === ''
-			|| !$comment->getVerb()
-		) {
-			throw new \UnexpectedValueException('Actor, Object and Verb information must be provided for saving');
-		}
-
-		if ($comment->getId() === '') {
-			$comment->setChildrenCount(0);
-			$comment->setLatestChildDateTime(new \DateTime('0000-00-00 00:00:00', new \DateTimeZone('UTC')));
-			$comment->setLatestChildDateTime(null);
-		}
-
-		if (is_null($comment->getCreationDateTime())) {
-			$comment->setCreationDateTime(new \DateTime());
-		}
-
-		if ($comment->getParentId() !== '0') {
-			$comment->setTopmostParentId($this->determineTopmostParentId($comment->getParentId()));
-		} else {
-			$comment->setTopmostParentId('0');
-		}
-
-		$this->cache($comment);
-
-		return $comment;
-	}
-
-	/**
-	 * returns the topmost parent id of a given comment identified by ID
-	 *
-	 * @param string $id
-	 * @return string
-	 * @throws NotFoundException
-	 */
-	protected function determineTopmostParentId($id) {
-		$comment = $this->get($id);
-		if ($comment->getParentId() === '0') {
-			return $comment->getId();
-		}
-
-		return $this->determineTopmostParentId($comment->getParentId());
-	}
-
-	/**
-	 * updates child information of a comment
-	 *
-	 * @param string $id
-	 * @param \DateTime $cDateTime the date time of the most recent child
-	 * @throws NotFoundException
-	 */
-	protected function updateChildrenInformation($id, \DateTime $cDateTime) {
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->select($qb->func()->count('id'))
-			->from('comments')
-			->where($qb->expr()->eq('parent_id', $qb->createParameter('id')))
-			->setParameter('id', $id);
-
-		$resultStatement = $query->execute();
-		$data = $resultStatement->fetch(\PDO::FETCH_NUM);
-		$resultStatement->closeCursor();
-		$children = (int)$data[0];
-
-		$comment = $this->get($id);
-		$comment->setChildrenCount($children);
-		$comment->setLatestChildDateTime($cDateTime);
-		$this->save($comment);
-	}
-
-	/**
-	 * Tests whether actor or object type and id parameters are acceptable.
-	 * Throws exception if not.
-	 *
-	 * @param string $role
-	 * @param string $type
-	 * @param string $id
-	 * @throws \InvalidArgumentException
-	 */
-	protected function checkRoleParameters($role, $type, $id) {
-		if (
-			!is_string($type) || empty($type)
-			|| !is_string($id) || empty($id)
-		) {
-			throw new \InvalidArgumentException($role . ' parameters must be string and not empty');
-		}
-	}
-
-	/**
-	 * run-time caches a comment
-	 *
-	 * @param IComment $comment
-	 */
-	protected function cache(IComment $comment) {
-		$id = $comment->getId();
-		if (empty($id)) {
-			return;
-		}
-		$this->commentsCache[(string)$id] = $comment;
-	}
-
-	/**
-	 * removes an entry from the comments run time cache
-	 *
-	 * @param mixed $id the comment's id
-	 */
-	protected function uncache($id) {
-		$id = (string)$id;
-		if (isset($this->commentsCache[$id])) {
-			unset($this->commentsCache[$id]);
-		}
-	}
-
-	/**
-	 * returns a comment instance
-	 *
-	 * @param string $id the ID of the comment
-	 * @return IComment
-	 * @throws NotFoundException
-	 * @throws \InvalidArgumentException
-	 * @since 9.0.0
-	 */
-	public function get($id) {
-		if ((int)$id === 0) {
-			throw new \InvalidArgumentException('IDs must be translatable to a number in this implementation.');
-		}
-
-		if (isset($this->commentsCache[$id])) {
-			return $this->commentsCache[$id];
-		}
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$resultStatement = $qb->select('*')
-			->from('comments')
-			->where($qb->expr()->eq('id', $qb->createParameter('id')))
-			->setParameter('id', $id, IQueryBuilder::PARAM_INT)
-			->execute();
-
-		$data = $resultStatement->fetch();
-		$resultStatement->closeCursor();
-		if (!$data) {
-			throw new NotFoundException();
-		}
-
-
-		$comment = $this->getCommentFromData($data);
-		$this->cache($comment);
-		return $comment;
-	}
-
-	/**
-	 * returns the comment specified by the id and all it's child comments.
-	 * At this point of time, we do only support one level depth.
-	 *
-	 * @param string $id
-	 * @param int $limit max number of entries to return, 0 returns all
-	 * @param int $offset the start entry
-	 * @return array
-	 * @since 9.0.0
-	 *
-	 * The return array looks like this
-	 * [
-	 *   'comment' => IComment, // root comment
-	 *   'replies' =>
-	 *   [
-	 *     0 =>
-	 *     [
-	 *       'comment' => IComment,
-	 *       'replies' => []
-	 *     ]
-	 *     1 =>
-	 *     [
-	 *       'comment' => IComment,
-	 *       'replies'=> []
-	 *     ],
-	 *     …
-	 *   ]
-	 * ]
-	 */
-	public function getTree($id, $limit = 0, $offset = 0) {
-		$tree = [];
-		$tree['comment'] = $this->get($id);
-		$tree['replies'] = [];
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->select('*')
-			->from('comments')
-			->where($qb->expr()->eq('topmost_parent_id', $qb->createParameter('id')))
-			->orderBy('creation_timestamp', 'DESC')
-			->setParameter('id', $id);
-
-		if ($limit > 0) {
-			$query->setMaxResults($limit);
-		}
-		if ($offset > 0) {
-			$query->setFirstResult($offset);
-		}
-
-		$resultStatement = $query->execute();
-		while ($data = $resultStatement->fetch()) {
-			$comment = $this->getCommentFromData($data);
-			$this->cache($comment);
-			$tree['replies'][] = [
-				'comment' => $comment,
-				'replies' => []
-			];
-		}
-		$resultStatement->closeCursor();
-
-		return $tree;
-	}
-
-	/**
-	 * returns comments for a specific object (e.g. a file).
-	 *
-	 * The sort order is always newest to oldest.
-	 *
-	 * @param string $objectType the object type, e.g. 'files'
-	 * @param string $objectId the id of the object
-	 * @param int $limit optional, number of maximum comments to be returned. if
-	 * not specified, all comments are returned.
-	 * @param int $offset optional, starting point
-	 * @param \DateTime $notOlderThan optional, timestamp of the oldest comments
-	 * that may be returned
-	 * @return IComment[]
-	 * @since 9.0.0
-	 */
-	public function getForObject(
-		$objectType,
-		$objectId,
-		$limit = 0,
-		$offset = 0,
-		\DateTime $notOlderThan = null
-	) {
-		$comments = [];
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->select('*')
-			->from('comments')
-			->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
-			->orderBy('creation_timestamp', 'DESC')
-			->setParameter('type', $objectType)
-			->setParameter('id', $objectId);
-
-		if ($limit > 0) {
-			$query->setMaxResults($limit);
-		}
-		if ($offset > 0) {
-			$query->setFirstResult($offset);
-		}
-		if (!is_null($notOlderThan)) {
-			$query
-				->andWhere($qb->expr()->gt('creation_timestamp', $qb->createParameter('notOlderThan')))
-				->setParameter('notOlderThan', $notOlderThan, 'datetime');
-		}
-
-		$resultStatement = $query->execute();
-		while ($data = $resultStatement->fetch()) {
-			$comment = $this->getCommentFromData($data);
-			$this->cache($comment);
-			$comments[] = $comment;
-		}
-		$resultStatement->closeCursor();
-
-		return $comments;
-	}
-
-	/**
-	 * @param string $objectType the object type, e.g. 'files'
-	 * @param string $objectId the id of the object
-	 * @param int $lastKnownCommentId the last known comment (will be used as offset)
-	 * @param string $sortDirection direction of the comments (`asc` or `desc`)
-	 * @param int $limit optional, number of maximum comments to be returned. if
-	 * set to 0, all comments are returned.
-	 * @param bool $includeLastKnown
-	 * @return IComment[]
-	 * @return array
-	 */
-	public function getForObjectSince(
-		string $objectType,
-		string $objectId,
-		int $lastKnownCommentId,
-		string $sortDirection = 'asc',
-		int $limit = 30,
-		bool $includeLastKnown = false
-	): array {
-		$comments = [];
-
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select('*')
-			->from('comments')
-			->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
-			->orderBy('creation_timestamp', $sortDirection === 'desc' ? 'DESC' : 'ASC')
-			->addOrderBy('id', $sortDirection === 'desc' ? 'DESC' : 'ASC');
-
-		if ($limit > 0) {
-			$query->setMaxResults($limit);
-		}
-
-		$lastKnownComment = $lastKnownCommentId > 0 ? $this->getLastKnownComment(
-			$objectType,
-			$objectId,
-			$lastKnownCommentId
-		) : null;
-		if ($lastKnownComment instanceof IComment) {
-			$lastKnownCommentDateTime = $lastKnownComment->getCreationDateTime();
-			if ($sortDirection === 'desc') {
-				if ($includeLastKnown) {
-					$idComparison = $query->expr()->lte('id', $query->createNamedParameter($lastKnownCommentId));
-				} else {
-					$idComparison = $query->expr()->lt('id', $query->createNamedParameter($lastKnownCommentId));
-				}
-				$query->andWhere(
-					$query->expr()->orX(
-						$query->expr()->lt(
-							'creation_timestamp',
-							$query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
-							IQueryBuilder::PARAM_DATE
-						),
-						$query->expr()->andX(
-							$query->expr()->eq(
-								'creation_timestamp',
-								$query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
-								IQueryBuilder::PARAM_DATE
-							),
-							$idComparison
-						)
-					)
-				);
-			} else {
-				if ($includeLastKnown) {
-					$idComparison = $query->expr()->gte('id', $query->createNamedParameter($lastKnownCommentId));
-				} else {
-					$idComparison = $query->expr()->gt('id', $query->createNamedParameter($lastKnownCommentId));
-				}
-				$query->andWhere(
-					$query->expr()->orX(
-						$query->expr()->gt(
-							'creation_timestamp',
-							$query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
-							IQueryBuilder::PARAM_DATE
-						),
-						$query->expr()->andX(
-							$query->expr()->eq(
-								'creation_timestamp',
-								$query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
-								IQueryBuilder::PARAM_DATE
-							),
-							$idComparison
-						)
-					)
-				);
-			}
-		}
-
-		$resultStatement = $query->execute();
-		while ($data = $resultStatement->fetch()) {
-			$comment = $this->getCommentFromData($data);
-			$this->cache($comment);
-			$comments[] = $comment;
-		}
-		$resultStatement->closeCursor();
-
-		return $comments;
-	}
-
-	/**
-	 * @param string $objectType the object type, e.g. 'files'
-	 * @param string $objectId the id of the object
-	 * @param int $id the comment to look for
-	 * @return Comment|null
-	 */
-	protected function getLastKnownComment(string $objectType,
-										   string $objectId,
-										   int $id) {
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select('*')
-			->from('comments')
-			->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
-			->andWhere($query->expr()->eq('id', $query->createNamedParameter($id, IQueryBuilder::PARAM_INT)));
-
-		$result = $query->execute();
-		$row = $result->fetch();
-		$result->closeCursor();
-
-		if ($row) {
-			$comment = $this->getCommentFromData($row);
-			$this->cache($comment);
-			return $comment;
-		}
-
-		return null;
-	}
-
-	/**
-	 * Search for comments with a given content
-	 *
-	 * @param string $search content to search for
-	 * @param string $objectType Limit the search by object type
-	 * @param string $objectId Limit the search by object id
-	 * @param string $verb Limit the verb of the comment
-	 * @param int $offset
-	 * @param int $limit
-	 * @return IComment[]
-	 */
-	public function search(string $search, string $objectType, string $objectId, string $verb, int $offset, int $limit = 50): array {
-		$objectIds = [];
-		if ($objectId) {
-			$objectIds[] = $objectIds;
-		}
-		return $this->searchForObjects($search, $objectType, $objectIds, $verb, $offset, $limit);
-	}
-
-	/**
-	 * Search for comments on one or more objects with a given content
-	 *
-	 * @param string $search content to search for
-	 * @param string $objectType Limit the search by object type
-	 * @param array $objectIds Limit the search by object ids
-	 * @param string $verb Limit the verb of the comment
-	 * @param int $offset
-	 * @param int $limit
-	 * @return IComment[]
-	 */
-	public function searchForObjects(string $search, string $objectType, array $objectIds, string $verb, int $offset, int $limit = 50): array {
-		$query = $this->dbConn->getQueryBuilder();
-
-		$query->select('*')
-			->from('comments')
-			->where($query->expr()->iLike('message', $query->createNamedParameter(
-				'%' . $this->dbConn->escapeLikeParameter($search). '%'
-			)))
-			->orderBy('creation_timestamp', 'DESC')
-			->addOrderBy('id', 'DESC')
-			->setMaxResults($limit);
-
-		if ($objectType !== '') {
-			$query->andWhere($query->expr()->eq('object_type', $query->createNamedParameter($objectType)));
-		}
-		if (!empty($objectIds)) {
-			$query->andWhere($query->expr()->in('object_id', $query->createNamedParameter($objectIds, IQueryBuilder::PARAM_STR_ARRAY)));
-		}
-		if ($verb !== '') {
-			$query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
-		}
-		if ($offset !== 0) {
-			$query->setFirstResult($offset);
-		}
-
-		$comments = [];
-		$result = $query->execute();
-		while ($data = $result->fetch()) {
-			$comment = $this->getCommentFromData($data);
-			$this->cache($comment);
-			$comments[] = $comment;
-		}
-		$result->closeCursor();
-
-		return $comments;
-	}
-
-	/**
-	 * @param $objectType string the object type, e.g. 'files'
-	 * @param $objectId string the id of the object
-	 * @param \DateTime $notOlderThan optional, timestamp of the oldest comments
-	 * that may be returned
-	 * @param string $verb Limit the verb of the comment - Added in 14.0.0
-	 * @return Int
-	 * @since 9.0.0
-	 */
-	public function getNumberOfCommentsForObject($objectType, $objectId, \DateTime $notOlderThan = null, $verb = '') {
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->select($qb->func()->count('id'))
-			->from('comments')
-			->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
-			->setParameter('type', $objectType)
-			->setParameter('id', $objectId);
-
-		if (!is_null($notOlderThan)) {
-			$query
-				->andWhere($qb->expr()->gt('creation_timestamp', $qb->createParameter('notOlderThan')))
-				->setParameter('notOlderThan', $notOlderThan, 'datetime');
-		}
-
-		if ($verb !== '') {
-			$query->andWhere($qb->expr()->eq('verb', $qb->createNamedParameter($verb)));
-		}
-
-		$resultStatement = $query->execute();
-		$data = $resultStatement->fetch(\PDO::FETCH_NUM);
-		$resultStatement->closeCursor();
-		return (int)$data[0];
-	}
-
-	/**
-	 * @param string $objectType the object type, e.g. 'files'
-	 * @param string[] $objectIds the id of the object
-	 * @param IUser $user
-	 * @param string $verb Limit the verb of the comment - Added in 14.0.0
-	 * @return array Map with object id => # of unread comments
-	 * @psalm-return array<string, int>
-	 * @since 21.0.0
-	 */
-	public function getNumberOfUnreadCommentsForObjects(string $objectType, array $objectIds, IUser $user, $verb = ''): array {
-		$unreadComments = [];
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select('c.object_id', $query->func()->count('c.id', 'num_comments'))
-			->from('comments', 'c')
-			->leftJoin('c', 'comments_read_markers', 'm', $query->expr()->andX(
-				$query->expr()->eq('m.user_id', $query->createNamedParameter($user->getUID())),
-				$query->expr()->eq('c.object_type', 'm.object_type'),
-				$query->expr()->eq('c.object_id', 'm.object_id')
-			))
-			->where($query->expr()->eq('c.object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->in('c.object_id', $query->createParameter('ids')))
-			->andWhere($query->expr()->orX(
-				$query->expr()->gt('c.creation_timestamp', 'm.marker_datetime'),
-				$query->expr()->isNull('m.marker_datetime')
-			))
-			->groupBy('c.object_id');
-
-		if ($verb !== '') {
-			$query->andWhere($query->expr()->eq('c.verb', $query->createNamedParameter($verb)));
-		}
-
-		$unreadComments = array_fill_keys($objectIds, 0);
-		foreach (array_chunk($objectIds, 1000) as $chunk) {
-			$query->setParameter('ids', $chunk, IQueryBuilder::PARAM_INT_ARRAY);
-
-			$result = $query->executeQuery();
-			while ($row = $result->fetch()) {
-				$unreadComments[$row['object_id']] = (int) $row['num_comments'];
-			}
-			$result->closeCursor();
-		}
-
-		return $unreadComments;
-	}
-
-	/**
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @param int $lastRead
-	 * @param string $verb
-	 * @return int
-	 * @since 21.0.0
-	 */
-	public function getNumberOfCommentsForObjectSinceComment(string $objectType, string $objectId, int $lastRead, string $verb = ''): int {
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select($query->func()->count('id', 'num_messages'))
-			->from('comments')
-			->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
-			->andWhere($query->expr()->gt('id', $query->createNamedParameter($lastRead)));
-
-		if ($verb !== '') {
-			$query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
-		}
-
-		$result = $query->execute();
-		$data = $result->fetch();
-		$result->closeCursor();
-
-		return (int) ($data['num_messages'] ?? 0);
-	}
-
-	/**
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @param \DateTime $beforeDate
-	 * @param string $verb
-	 * @return int
-	 * @since 21.0.0
-	 */
-	public function getLastCommentBeforeDate(string $objectType, string $objectId, \DateTime $beforeDate, string $verb = ''): int {
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select('id')
-			->from('comments')
-			->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
-			->andWhere($query->expr()->lt('creation_timestamp', $query->createNamedParameter($beforeDate, IQueryBuilder::PARAM_DATE)))
-			->orderBy('creation_timestamp', 'desc');
-
-		if ($verb !== '') {
-			$query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
-		}
-
-		$result = $query->execute();
-		$data = $result->fetch();
-		$result->closeCursor();
-
-		return (int) ($data['id'] ?? 0);
-	}
-
-	/**
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @param string $verb
-	 * @param string $actorType
-	 * @param string[] $actors
-	 * @return \DateTime[] Map of "string actor" => "\DateTime most recent comment date"
-	 * @psalm-return array<string, \DateTime>
-	 * @since 21.0.0
-	 */
-	public function getLastCommentDateByActor(
-		string $objectType,
-		string $objectId,
-		string $verb,
-		string $actorType,
-		array $actors
-	): array {
-		$lastComments = [];
-
-		$query = $this->dbConn->getQueryBuilder();
-		$query->select('actor_id')
-			->selectAlias($query->createFunction('MAX(' . $query->getColumnName('creation_timestamp') . ')'), 'last_comment')
-			->from('comments')
-			->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
-			->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
-			->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)))
-			->andWhere($query->expr()->eq('actor_type', $query->createNamedParameter($actorType)))
-			->andWhere($query->expr()->in('actor_id', $query->createNamedParameter($actors, IQueryBuilder::PARAM_STR_ARRAY)))
-			->groupBy('actor_id');
-
-		$result = $query->execute();
-		while ($row = $result->fetch()) {
-			$lastComments[$row['actor_id']] = $this->timeFactory->getDateTime($row['last_comment']);
-		}
-		$result->closeCursor();
-
-		return $lastComments;
-	}
-
-	/**
-	 * Get the number of unread comments for all files in a folder
-	 *
-	 * @param int $folderId
-	 * @param IUser $user
-	 * @return array [$fileId => $unreadCount]
-	 */
-	public function getNumberOfUnreadCommentsForFolder($folderId, IUser $user) {
-		$qb = $this->dbConn->getQueryBuilder();
-
-		$query = $qb->select('f.fileid')
-			->addSelect($qb->func()->count('c.id', 'num_ids'))
-			->from('filecache', 'f')
-			->leftJoin('f', 'comments', 'c', $qb->expr()->andX(
-				$qb->expr()->eq('f.fileid', $qb->expr()->castColumn('c.object_id', IQueryBuilder::PARAM_INT)),
-				$qb->expr()->eq('c.object_type', $qb->createNamedParameter('files'))
-			))
-			->leftJoin('c', 'comments_read_markers', 'm', $qb->expr()->andX(
-				$qb->expr()->eq('c.object_id', 'm.object_id'),
-				$qb->expr()->eq('m.object_type', $qb->createNamedParameter('files'))
-			))
-			->where(
-				$qb->expr()->andX(
-					$qb->expr()->eq('f.parent', $qb->createNamedParameter($folderId)),
-					$qb->expr()->orX(
-						$qb->expr()->eq('c.object_type', $qb->createNamedParameter('files')),
-						$qb->expr()->isNull('c.object_type')
-					),
-					$qb->expr()->orX(
-						$qb->expr()->eq('m.object_type', $qb->createNamedParameter('files')),
-						$qb->expr()->isNull('m.object_type')
-					),
-					$qb->expr()->orX(
-						$qb->expr()->eq('m.user_id', $qb->createNamedParameter($user->getUID())),
-						$qb->expr()->isNull('m.user_id')
-					),
-					$qb->expr()->orX(
-						$qb->expr()->gt('c.creation_timestamp', 'm.marker_datetime'),
-						$qb->expr()->isNull('m.marker_datetime')
-					)
-				)
-			)->groupBy('f.fileid');
-
-		$resultStatement = $query->execute();
-
-		$results = [];
-		while ($row = $resultStatement->fetch()) {
-			$results[$row['fileid']] = (int) $row['num_ids'];
-		}
-		$resultStatement->closeCursor();
-		return $results;
-	}
-
-	/**
-	 * creates a new comment and returns it. At this point of time, it is not
-	 * saved in the used data storage. Use save() after setting other fields
-	 * of the comment (e.g. message or verb).
-	 *
-	 * @param string $actorType the actor type (e.g. 'users')
-	 * @param string $actorId a user id
-	 * @param string $objectType the object type the comment is attached to
-	 * @param string $objectId the object id the comment is attached to
-	 * @return IComment
-	 * @since 9.0.0
-	 */
-	public function create($actorType, $actorId, $objectType, $objectId) {
-		$comment = new Comment();
-		$comment
-			->setActor($actorType, $actorId)
-			->setObject($objectType, $objectId);
-		return $comment;
-	}
-
-	/**
-	 * permanently deletes the comment specified by the ID
-	 *
-	 * When the comment has child comments, their parent ID will be changed to
-	 * the parent ID of the item that is to be deleted.
-	 *
-	 * @param string $id
-	 * @return bool
-	 * @throws \InvalidArgumentException
-	 * @since 9.0.0
-	 */
-	public function delete($id) {
-		if (!is_string($id)) {
-			throw new \InvalidArgumentException('Parameter must be string');
-		}
-
-		try {
-			$comment = $this->get($id);
-		} catch (\Exception $e) {
-			// Ignore exceptions, we just don't fire a hook then
-			$comment = null;
-		}
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->delete('comments')
-			->where($qb->expr()->eq('id', $qb->createParameter('id')))
-			->setParameter('id', $id);
-
-		try {
-			$affectedRows = $query->execute();
-			$this->uncache($id);
-		} catch (DriverException $e) {
-			$this->logger->error($e->getMessage(), [
-				'exception' => $e,
-				'app' => 'core_comments',
-			]);
-			return false;
-		}
-
-		if ($affectedRows > 0 && $comment instanceof IComment) {
-			$this->sendEvent(CommentsEvent::EVENT_DELETE, $comment);
-		}
-
-		return ($affectedRows > 0);
-	}
-
-	/**
-	 * saves the comment permanently
-	 *
-	 * if the supplied comment has an empty ID, a new entry comment will be
-	 * saved and the instance updated with the new ID.
-	 *
-	 * Otherwise, an existing comment will be updated.
-	 *
-	 * Throws NotFoundException when a comment that is to be updated does not
-	 * exist anymore at this point of time.
-	 *
-	 * @param IComment $comment
-	 * @return bool
-	 * @throws NotFoundException
-	 * @since 9.0.0
-	 */
-	public function save(IComment $comment) {
-		if ($this->prepareCommentForDatabaseWrite($comment)->getId() === '') {
-			$result = $this->insert($comment);
-		} else {
-			$result = $this->update($comment);
-		}
-
-		if ($result && !!$comment->getParentId()) {
-			$this->updateChildrenInformation(
-				$comment->getParentId(),
-				$comment->getCreationDateTime()
-			);
-			$this->cache($comment);
-		}
-
-		return $result;
-	}
-
-	/**
-	 * inserts the provided comment in the database
-	 *
-	 * @param IComment $comment
-	 * @return bool
-	 */
-	protected function insert(IComment $comment): bool {
-		try {
-			$result = $this->insertQuery($comment, true);
-		} catch (InvalidFieldNameException $e) {
-			// The reference id field was only added in Nextcloud 19.
-			// In order to not cause too long waiting times on the update,
-			// it was decided to only add it lazy, as it is also not a critical
-			// feature, but only helps to have a better experience while commenting.
-			// So in case the reference_id field is missing,
-			// we simply save the comment without that field.
-			$result = $this->insertQuery($comment, false);
-		}
-
-		return $result;
-	}
-
-	protected function insertQuery(IComment $comment, bool $tryWritingReferenceId): bool {
-		$qb = $this->dbConn->getQueryBuilder();
-
-		$values = [
-			'parent_id' => $qb->createNamedParameter($comment->getParentId()),
-			'topmost_parent_id' => $qb->createNamedParameter($comment->getTopmostParentId()),
-			'children_count' => $qb->createNamedParameter($comment->getChildrenCount()),
-			'actor_type' => $qb->createNamedParameter($comment->getActorType()),
-			'actor_id' => $qb->createNamedParameter($comment->getActorId()),
-			'message' => $qb->createNamedParameter($comment->getMessage()),
-			'verb' => $qb->createNamedParameter($comment->getVerb()),
-			'creation_timestamp' => $qb->createNamedParameter($comment->getCreationDateTime(), 'datetime'),
-			'latest_child_timestamp' => $qb->createNamedParameter($comment->getLatestChildDateTime(), 'datetime'),
-			'object_type' => $qb->createNamedParameter($comment->getObjectType()),
-			'object_id' => $qb->createNamedParameter($comment->getObjectId()),
-		];
-
-		if ($tryWritingReferenceId) {
-			$values['reference_id'] = $qb->createNamedParameter($comment->getReferenceId());
-		}
-
-		$affectedRows = $qb->insert('comments')
-			->values($values)
-			->execute();
-
-		if ($affectedRows > 0) {
-			$comment->setId((string)$qb->getLastInsertId());
-			$this->sendEvent(CommentsEvent::EVENT_ADD, $comment);
-		}
-
-		return $affectedRows > 0;
-	}
-
-	/**
-	 * updates a Comment data row
-	 *
-	 * @param IComment $comment
-	 * @return bool
-	 * @throws NotFoundException
-	 */
-	protected function update(IComment $comment) {
-		// for properly working preUpdate Events we need the old comments as is
-		// in the DB and overcome caching. Also avoid that outdated information stays.
-		$this->uncache($comment->getId());
-		$this->sendEvent(CommentsEvent::EVENT_PRE_UPDATE, $this->get($comment->getId()));
-		$this->uncache($comment->getId());
-
-		try {
-			$result = $this->updateQuery($comment, true);
-		} catch (InvalidFieldNameException $e) {
-			// See function insert() for explanation
-			$result = $this->updateQuery($comment, false);
-		}
-
-		$this->sendEvent(CommentsEvent::EVENT_UPDATE, $comment);
-
-		return $result;
-	}
-
-	protected function updateQuery(IComment $comment, bool $tryWritingReferenceId): bool {
-		$qb = $this->dbConn->getQueryBuilder();
-		$qb
-			->update('comments')
-			->set('parent_id', $qb->createNamedParameter($comment->getParentId()))
-			->set('topmost_parent_id', $qb->createNamedParameter($comment->getTopmostParentId()))
-			->set('children_count', $qb->createNamedParameter($comment->getChildrenCount()))
-			->set('actor_type', $qb->createNamedParameter($comment->getActorType()))
-			->set('actor_id', $qb->createNamedParameter($comment->getActorId()))
-			->set('message', $qb->createNamedParameter($comment->getMessage()))
-			->set('verb', $qb->createNamedParameter($comment->getVerb()))
-			->set('creation_timestamp', $qb->createNamedParameter($comment->getCreationDateTime(), 'datetime'))
-			->set('latest_child_timestamp', $qb->createNamedParameter($comment->getLatestChildDateTime(), 'datetime'))
-			->set('object_type', $qb->createNamedParameter($comment->getObjectType()))
-			->set('object_id', $qb->createNamedParameter($comment->getObjectId()));
-
-		if ($tryWritingReferenceId) {
-			$qb->set('reference_id', $qb->createNamedParameter($comment->getReferenceId()));
-		}
-
-		$affectedRows = $qb->where($qb->expr()->eq('id', $qb->createNamedParameter($comment->getId())))
-			->execute();
-
-		if ($affectedRows === 0) {
-			throw new NotFoundException('Comment to update does ceased to exist');
-		}
-
-		return $affectedRows > 0;
-	}
-
-	/**
-	 * removes references to specific actor (e.g. on user delete) of a comment.
-	 * The comment itself must not get lost/deleted.
-	 *
-	 * @param string $actorType the actor type (e.g. 'users')
-	 * @param string $actorId a user id
-	 * @return boolean
-	 * @since 9.0.0
-	 */
-	public function deleteReferencesOfActor($actorType, $actorId) {
-		$this->checkRoleParameters('Actor', $actorType, $actorId);
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$affectedRows = $qb
-			->update('comments')
-			->set('actor_type', $qb->createNamedParameter(ICommentsManager::DELETED_USER))
-			->set('actor_id', $qb->createNamedParameter(ICommentsManager::DELETED_USER))
-			->where($qb->expr()->eq('actor_type', $qb->createParameter('type')))
-			->andWhere($qb->expr()->eq('actor_id', $qb->createParameter('id')))
-			->setParameter('type', $actorType)
-			->setParameter('id', $actorId)
-			->execute();
-
-		$this->commentsCache = [];
-
-		return is_int($affectedRows);
-	}
-
-	/**
-	 * deletes all comments made of a specific object (e.g. on file delete)
-	 *
-	 * @param string $objectType the object type (e.g. 'files')
-	 * @param string $objectId e.g. the file id
-	 * @return boolean
-	 * @since 9.0.0
-	 */
-	public function deleteCommentsAtObject($objectType, $objectId) {
-		$this->checkRoleParameters('Object', $objectType, $objectId);
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$affectedRows = $qb
-			->delete('comments')
-			->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
-			->setParameter('type', $objectType)
-			->setParameter('id', $objectId)
-			->execute();
-
-		$this->commentsCache = [];
-
-		return is_int($affectedRows);
-	}
-
-	/**
-	 * deletes the read markers for the specified user
-	 *
-	 * @param \OCP\IUser $user
-	 * @return bool
-	 * @since 9.0.0
-	 */
-	public function deleteReadMarksFromUser(IUser $user) {
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->delete('comments_read_markers')
-			->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
-			->setParameter('user_id', $user->getUID());
-
-		try {
-			$affectedRows = $query->execute();
-		} catch (DriverException $e) {
-			$this->logger->error($e->getMessage(), [
-				'exception' => $e,
-				'app' => 'core_comments',
-			]);
-			return false;
-		}
-		return ($affectedRows > 0);
-	}
-
-	/**
-	 * sets the read marker for a given file to the specified date for the
-	 * provided user
-	 *
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @param \DateTime $dateTime
-	 * @param IUser $user
-	 * @since 9.0.0
-	 */
-	public function setReadMark($objectType, $objectId, \DateTime $dateTime, IUser $user) {
-		$this->checkRoleParameters('Object', $objectType, $objectId);
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$values = [
-			'user_id' => $qb->createNamedParameter($user->getUID()),
-			'marker_datetime' => $qb->createNamedParameter($dateTime, 'datetime'),
-			'object_type' => $qb->createNamedParameter($objectType),
-			'object_id' => $qb->createNamedParameter($objectId),
-		];
-
-		// Strategy: try to update, if this does not return affected rows, do an insert.
-		$affectedRows = $qb
-			->update('comments_read_markers')
-			->set('user_id', $values['user_id'])
-			->set('marker_datetime', $values['marker_datetime'])
-			->set('object_type', $values['object_type'])
-			->set('object_id', $values['object_id'])
-			->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
-			->andWhere($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
-			->setParameter('user_id', $user->getUID(), IQueryBuilder::PARAM_STR)
-			->setParameter('object_type', $objectType, IQueryBuilder::PARAM_STR)
-			->setParameter('object_id', $objectId, IQueryBuilder::PARAM_STR)
-			->execute();
-
-		if ($affectedRows > 0) {
-			return;
-		}
-
-		$qb->insert('comments_read_markers')
-			->values($values)
-			->execute();
-	}
-
-	/**
-	 * returns the read marker for a given file to the specified date for the
-	 * provided user. It returns null, when the marker is not present, i.e.
-	 * no comments were marked as read.
-	 *
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @param IUser $user
-	 * @return \DateTime|null
-	 * @since 9.0.0
-	 */
-	public function getReadMark($objectType, $objectId, IUser $user) {
-		$qb = $this->dbConn->getQueryBuilder();
-		$resultStatement = $qb->select('marker_datetime')
-			->from('comments_read_markers')
-			->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
-			->andWhere($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
-			->setParameter('user_id', $user->getUID(), IQueryBuilder::PARAM_STR)
-			->setParameter('object_type', $objectType, IQueryBuilder::PARAM_STR)
-			->setParameter('object_id', $objectId, IQueryBuilder::PARAM_STR)
-			->execute();
-
-		$data = $resultStatement->fetch();
-		$resultStatement->closeCursor();
-		if (!$data || is_null($data['marker_datetime'])) {
-			return null;
-		}
-
-		return new \DateTime($data['marker_datetime']);
-	}
-
-	/**
-	 * deletes the read markers on the specified object
-	 *
-	 * @param string $objectType
-	 * @param string $objectId
-	 * @return bool
-	 * @since 9.0.0
-	 */
-	public function deleteReadMarksOnObject($objectType, $objectId) {
-		$this->checkRoleParameters('Object', $objectType, $objectId);
-
-		$qb = $this->dbConn->getQueryBuilder();
-		$query = $qb->delete('comments_read_markers')
-			->where($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
-			->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
-			->setParameter('object_type', $objectType)
-			->setParameter('object_id', $objectId);
-
-		try {
-			$affectedRows = $query->execute();
-		} catch (DriverException $e) {
-			$this->logger->error($e->getMessage(), [
-				'exception' => $e,
-				'app' => 'core_comments',
-			]);
-			return false;
-		}
-		return ($affectedRows > 0);
-	}
-
-	/**
-	 * registers an Entity to the manager, so event notifications can be send
-	 * to consumers of the comments infrastructure
-	 *
-	 * @param \Closure $closure
-	 */
-	public function registerEventHandler(\Closure $closure) {
-		$this->eventHandlerClosures[] = $closure;
-		$this->eventHandlers = [];
-	}
-
-	/**
-	 * registers a method that resolves an ID to a display name for a given type
-	 *
-	 * @param string $type
-	 * @param \Closure $closure
-	 * @throws \OutOfBoundsException
-	 * @since 11.0.0
-	 *
-	 * Only one resolver shall be registered per type. Otherwise a
-	 * \OutOfBoundsException has to thrown.
-	 */
-	public function registerDisplayNameResolver($type, \Closure $closure) {
-		if (!is_string($type)) {
-			throw new \InvalidArgumentException('String expected.');
-		}
-		if (isset($this->displayNameResolvers[$type])) {
-			throw new \OutOfBoundsException('Displayname resolver for this type already registered');
-		}
-		$this->displayNameResolvers[$type] = $closure;
-	}
-
-	/**
-	 * resolves a given ID of a given Type to a display name.
-	 *
-	 * @param string $type
-	 * @param string $id
-	 * @return string
-	 * @throws \OutOfBoundsException
-	 * @since 11.0.0
-	 *
-	 * If a provided type was not registered, an \OutOfBoundsException shall
-	 * be thrown. It is upon the resolver discretion what to return of the
-	 * provided ID is unknown. It must be ensured that a string is returned.
-	 */
-	public function resolveDisplayName($type, $id) {
-		if (!is_string($type)) {
-			throw new \InvalidArgumentException('String expected.');
-		}
-		if (!isset($this->displayNameResolvers[$type])) {
-			throw new \OutOfBoundsException('No Displayname resolver for this type registered');
-		}
-		return (string)$this->displayNameResolvers[$type]($id);
-	}
-
-	/**
-	 * returns valid, registered entities
-	 *
-	 * @return \OCP\Comments\ICommentsEventHandler[]
-	 */
-	private function getEventHandlers() {
-		if (!empty($this->eventHandlers)) {
-			return $this->eventHandlers;
-		}
-
-		$this->eventHandlers = [];
-		foreach ($this->eventHandlerClosures as $name => $closure) {
-			$entity = $closure();
-			if (!($entity instanceof ICommentsEventHandler)) {
-				throw new \InvalidArgumentException('The given entity does not implement the ICommentsEntity interface');
-			}
-			$this->eventHandlers[$name] = $entity;
-		}
-
-		return $this->eventHandlers;
-	}
-
-	/**
-	 * sends notifications to the registered entities
-	 *
-	 * @param $eventType
-	 * @param IComment $comment
-	 */
-	private function sendEvent($eventType, IComment $comment) {
-		$entities = $this->getEventHandlers();
-		$event = new CommentsEvent($eventType, $comment);
-		foreach ($entities as $entity) {
-			$entity->handle($event);
-		}
-	}
-
-	/**
-	 * Load the Comments app into the page
-	 *
-	 * @since 21.0.0
-	 */
-	public function load(): void {
-		$this->initialStateService->provideInitialState('comments', 'max-message-length', IComment::MAX_MESSAGE_LENGTH);
-		Util::addScript('comments', 'comments-app');
-	}
+    /** @var  IDBConnection */
+    protected $dbConn;
+
+    /** @var  LoggerInterface */
+    protected $logger;
+
+    /** @var IConfig */
+    protected $config;
+
+    /** @var ITimeFactory */
+    protected $timeFactory;
+
+    /** @var IInitialStateService */
+    protected $initialStateService;
+
+    /** @var IComment[] */
+    protected $commentsCache = [];
+
+    /** @var  \Closure[] */
+    protected $eventHandlerClosures = [];
+
+    /** @var  ICommentsEventHandler[] */
+    protected $eventHandlers = [];
+
+    /** @var \Closure[] */
+    protected $displayNameResolvers = [];
+
+    public function __construct(IDBConnection $dbConn,
+                                LoggerInterface $logger,
+                                IConfig $config,
+                                ITimeFactory $timeFactory,
+                                IInitialStateService $initialStateService) {
+        $this->dbConn = $dbConn;
+        $this->logger = $logger;
+        $this->config = $config;
+        $this->timeFactory = $timeFactory;
+        $this->initialStateService = $initialStateService;
+    }
+
+    /**
+     * converts data base data into PHP native, proper types as defined by
+     * IComment interface.
+     *
+     * @param array $data
+     * @return array
+     */
+    protected function normalizeDatabaseData(array $data) {
+        $data['id'] = (string)$data['id'];
+        $data['parent_id'] = (string)$data['parent_id'];
+        $data['topmost_parent_id'] = (string)$data['topmost_parent_id'];
+        $data['creation_timestamp'] = new \DateTime($data['creation_timestamp']);
+        if (!is_null($data['latest_child_timestamp'])) {
+            $data['latest_child_timestamp'] = new \DateTime($data['latest_child_timestamp']);
+        }
+        $data['children_count'] = (int)$data['children_count'];
+        $data['reference_id'] = $data['reference_id'] ?? null;
+        return $data;
+    }
+
+
+    /**
+     * @param array $data
+     * @return IComment
+     */
+    public function getCommentFromData(array $data): IComment {
+        return new Comment($this->normalizeDatabaseData($data));
+    }
+
+    /**
+     * prepares a comment for an insert or update operation after making sure
+     * all necessary fields have a value assigned.
+     *
+     * @param IComment $comment
+     * @return IComment returns the same updated IComment instance as provided
+     *                  by parameter for convenience
+     * @throws \UnexpectedValueException
+     */
+    protected function prepareCommentForDatabaseWrite(IComment $comment) {
+        if (!$comment->getActorType()
+            || $comment->getActorId() === ''
+            || !$comment->getObjectType()
+            || $comment->getObjectId() === ''
+            || !$comment->getVerb()
+        ) {
+            throw new \UnexpectedValueException('Actor, Object and Verb information must be provided for saving');
+        }
+
+        if ($comment->getId() === '') {
+            $comment->setChildrenCount(0);
+            $comment->setLatestChildDateTime(new \DateTime('0000-00-00 00:00:00', new \DateTimeZone('UTC')));
+            $comment->setLatestChildDateTime(null);
+        }
+
+        if (is_null($comment->getCreationDateTime())) {
+            $comment->setCreationDateTime(new \DateTime());
+        }
+
+        if ($comment->getParentId() !== '0') {
+            $comment->setTopmostParentId($this->determineTopmostParentId($comment->getParentId()));
+        } else {
+            $comment->setTopmostParentId('0');
+        }
+
+        $this->cache($comment);
+
+        return $comment;
+    }
+
+    /**
+     * returns the topmost parent id of a given comment identified by ID
+     *
+     * @param string $id
+     * @return string
+     * @throws NotFoundException
+     */
+    protected function determineTopmostParentId($id) {
+        $comment = $this->get($id);
+        if ($comment->getParentId() === '0') {
+            return $comment->getId();
+        }
+
+        return $this->determineTopmostParentId($comment->getParentId());
+    }
+
+    /**
+     * updates child information of a comment
+     *
+     * @param string $id
+     * @param \DateTime $cDateTime the date time of the most recent child
+     * @throws NotFoundException
+     */
+    protected function updateChildrenInformation($id, \DateTime $cDateTime) {
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->select($qb->func()->count('id'))
+            ->from('comments')
+            ->where($qb->expr()->eq('parent_id', $qb->createParameter('id')))
+            ->setParameter('id', $id);
+
+        $resultStatement = $query->execute();
+        $data = $resultStatement->fetch(\PDO::FETCH_NUM);
+        $resultStatement->closeCursor();
+        $children = (int)$data[0];
+
+        $comment = $this->get($id);
+        $comment->setChildrenCount($children);
+        $comment->setLatestChildDateTime($cDateTime);
+        $this->save($comment);
+    }
+
+    /**
+     * Tests whether actor or object type and id parameters are acceptable.
+     * Throws exception if not.
+     *
+     * @param string $role
+     * @param string $type
+     * @param string $id
+     * @throws \InvalidArgumentException
+     */
+    protected function checkRoleParameters($role, $type, $id) {
+        if (
+            !is_string($type) || empty($type)
+            || !is_string($id) || empty($id)
+        ) {
+            throw new \InvalidArgumentException($role . ' parameters must be string and not empty');
+        }
+    }
+
+    /**
+     * run-time caches a comment
+     *
+     * @param IComment $comment
+     */
+    protected function cache(IComment $comment) {
+        $id = $comment->getId();
+        if (empty($id)) {
+            return;
+        }
+        $this->commentsCache[(string)$id] = $comment;
+    }
+
+    /**
+     * removes an entry from the comments run time cache
+     *
+     * @param mixed $id the comment's id
+     */
+    protected function uncache($id) {
+        $id = (string)$id;
+        if (isset($this->commentsCache[$id])) {
+            unset($this->commentsCache[$id]);
+        }
+    }
+
+    /**
+     * returns a comment instance
+     *
+     * @param string $id the ID of the comment
+     * @return IComment
+     * @throws NotFoundException
+     * @throws \InvalidArgumentException
+     * @since 9.0.0
+     */
+    public function get($id) {
+        if ((int)$id === 0) {
+            throw new \InvalidArgumentException('IDs must be translatable to a number in this implementation.');
+        }
+
+        if (isset($this->commentsCache[$id])) {
+            return $this->commentsCache[$id];
+        }
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $resultStatement = $qb->select('*')
+            ->from('comments')
+            ->where($qb->expr()->eq('id', $qb->createParameter('id')))
+            ->setParameter('id', $id, IQueryBuilder::PARAM_INT)
+            ->execute();
+
+        $data = $resultStatement->fetch();
+        $resultStatement->closeCursor();
+        if (!$data) {
+            throw new NotFoundException();
+        }
+
+
+        $comment = $this->getCommentFromData($data);
+        $this->cache($comment);
+        return $comment;
+    }
+
+    /**
+     * returns the comment specified by the id and all it's child comments.
+     * At this point of time, we do only support one level depth.
+     *
+     * @param string $id
+     * @param int $limit max number of entries to return, 0 returns all
+     * @param int $offset the start entry
+     * @return array
+     * @since 9.0.0
+     *
+     * The return array looks like this
+     * [
+     *   'comment' => IComment, // root comment
+     *   'replies' =>
+     *   [
+     *     0 =>
+     *     [
+     *       'comment' => IComment,
+     *       'replies' => []
+     *     ]
+     *     1 =>
+     *     [
+     *       'comment' => IComment,
+     *       'replies'=> []
+     *     ],
+     *     …
+     *   ]
+     * ]
+     */
+    public function getTree($id, $limit = 0, $offset = 0) {
+        $tree = [];
+        $tree['comment'] = $this->get($id);
+        $tree['replies'] = [];
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->select('*')
+            ->from('comments')
+            ->where($qb->expr()->eq('topmost_parent_id', $qb->createParameter('id')))
+            ->orderBy('creation_timestamp', 'DESC')
+            ->setParameter('id', $id);
+
+        if ($limit > 0) {
+            $query->setMaxResults($limit);
+        }
+        if ($offset > 0) {
+            $query->setFirstResult($offset);
+        }
+
+        $resultStatement = $query->execute();
+        while ($data = $resultStatement->fetch()) {
+            $comment = $this->getCommentFromData($data);
+            $this->cache($comment);
+            $tree['replies'][] = [
+                'comment' => $comment,
+                'replies' => []
+            ];
+        }
+        $resultStatement->closeCursor();
+
+        return $tree;
+    }
+
+    /**
+     * returns comments for a specific object (e.g. a file).
+     *
+     * The sort order is always newest to oldest.
+     *
+     * @param string $objectType the object type, e.g. 'files'
+     * @param string $objectId the id of the object
+     * @param int $limit optional, number of maximum comments to be returned. if
+     * not specified, all comments are returned.
+     * @param int $offset optional, starting point
+     * @param \DateTime $notOlderThan optional, timestamp of the oldest comments
+     * that may be returned
+     * @return IComment[]
+     * @since 9.0.0
+     */
+    public function getForObject(
+        $objectType,
+        $objectId,
+        $limit = 0,
+        $offset = 0,
+        \DateTime $notOlderThan = null
+    ) {
+        $comments = [];
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->select('*')
+            ->from('comments')
+            ->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
+            ->orderBy('creation_timestamp', 'DESC')
+            ->setParameter('type', $objectType)
+            ->setParameter('id', $objectId);
+
+        if ($limit > 0) {
+            $query->setMaxResults($limit);
+        }
+        if ($offset > 0) {
+            $query->setFirstResult($offset);
+        }
+        if (!is_null($notOlderThan)) {
+            $query
+                ->andWhere($qb->expr()->gt('creation_timestamp', $qb->createParameter('notOlderThan')))
+                ->setParameter('notOlderThan', $notOlderThan, 'datetime');
+        }
+
+        $resultStatement = $query->execute();
+        while ($data = $resultStatement->fetch()) {
+            $comment = $this->getCommentFromData($data);
+            $this->cache($comment);
+            $comments[] = $comment;
+        }
+        $resultStatement->closeCursor();
+
+        return $comments;
+    }
+
+    /**
+     * @param string $objectType the object type, e.g. 'files'
+     * @param string $objectId the id of the object
+     * @param int $lastKnownCommentId the last known comment (will be used as offset)
+     * @param string $sortDirection direction of the comments (`asc` or `desc`)
+     * @param int $limit optional, number of maximum comments to be returned. if
+     * set to 0, all comments are returned.
+     * @param bool $includeLastKnown
+     * @return IComment[]
+     * @return array
+     */
+    public function getForObjectSince(
+        string $objectType,
+        string $objectId,
+        int $lastKnownCommentId,
+        string $sortDirection = 'asc',
+        int $limit = 30,
+        bool $includeLastKnown = false
+    ): array {
+        $comments = [];
+
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select('*')
+            ->from('comments')
+            ->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
+            ->orderBy('creation_timestamp', $sortDirection === 'desc' ? 'DESC' : 'ASC')
+            ->addOrderBy('id', $sortDirection === 'desc' ? 'DESC' : 'ASC');
+
+        if ($limit > 0) {
+            $query->setMaxResults($limit);
+        }
+
+        $lastKnownComment = $lastKnownCommentId > 0 ? $this->getLastKnownComment(
+            $objectType,
+            $objectId,
+            $lastKnownCommentId
+        ) : null;
+        if ($lastKnownComment instanceof IComment) {
+            $lastKnownCommentDateTime = $lastKnownComment->getCreationDateTime();
+            if ($sortDirection === 'desc') {
+                if ($includeLastKnown) {
+                    $idComparison = $query->expr()->lte('id', $query->createNamedParameter($lastKnownCommentId));
+                } else {
+                    $idComparison = $query->expr()->lt('id', $query->createNamedParameter($lastKnownCommentId));
+                }
+                $query->andWhere(
+                    $query->expr()->orX(
+                        $query->expr()->lt(
+                            'creation_timestamp',
+                            $query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
+                            IQueryBuilder::PARAM_DATE
+                        ),
+                        $query->expr()->andX(
+                            $query->expr()->eq(
+                                'creation_timestamp',
+                                $query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
+                                IQueryBuilder::PARAM_DATE
+                            ),
+                            $idComparison
+                        )
+                    )
+                );
+            } else {
+                if ($includeLastKnown) {
+                    $idComparison = $query->expr()->gte('id', $query->createNamedParameter($lastKnownCommentId));
+                } else {
+                    $idComparison = $query->expr()->gt('id', $query->createNamedParameter($lastKnownCommentId));
+                }
+                $query->andWhere(
+                    $query->expr()->orX(
+                        $query->expr()->gt(
+                            'creation_timestamp',
+                            $query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
+                            IQueryBuilder::PARAM_DATE
+                        ),
+                        $query->expr()->andX(
+                            $query->expr()->eq(
+                                'creation_timestamp',
+                                $query->createNamedParameter($lastKnownCommentDateTime, IQueryBuilder::PARAM_DATE),
+                                IQueryBuilder::PARAM_DATE
+                            ),
+                            $idComparison
+                        )
+                    )
+                );
+            }
+        }
+
+        $resultStatement = $query->execute();
+        while ($data = $resultStatement->fetch()) {
+            $comment = $this->getCommentFromData($data);
+            $this->cache($comment);
+            $comments[] = $comment;
+        }
+        $resultStatement->closeCursor();
+
+        return $comments;
+    }
+
+    /**
+     * @param string $objectType the object type, e.g. 'files'
+     * @param string $objectId the id of the object
+     * @param int $id the comment to look for
+     * @return Comment|null
+     */
+    protected function getLastKnownComment(string $objectType,
+                                            string $objectId,
+                                            int $id) {
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select('*')
+            ->from('comments')
+            ->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
+            ->andWhere($query->expr()->eq('id', $query->createNamedParameter($id, IQueryBuilder::PARAM_INT)));
+
+        $result = $query->execute();
+        $row = $result->fetch();
+        $result->closeCursor();
+
+        if ($row) {
+            $comment = $this->getCommentFromData($row);
+            $this->cache($comment);
+            return $comment;
+        }
+
+        return null;
+    }
+
+    /**
+     * Search for comments with a given content
+     *
+     * @param string $search content to search for
+     * @param string $objectType Limit the search by object type
+     * @param string $objectId Limit the search by object id
+     * @param string $verb Limit the verb of the comment
+     * @param int $offset
+     * @param int $limit
+     * @return IComment[]
+     */
+    public function search(string $search, string $objectType, string $objectId, string $verb, int $offset, int $limit = 50): array {
+        $objectIds = [];
+        if ($objectId) {
+            $objectIds[] = $objectIds;
+        }
+        return $this->searchForObjects($search, $objectType, $objectIds, $verb, $offset, $limit);
+    }
+
+    /**
+     * Search for comments on one or more objects with a given content
+     *
+     * @param string $search content to search for
+     * @param string $objectType Limit the search by object type
+     * @param array $objectIds Limit the search by object ids
+     * @param string $verb Limit the verb of the comment
+     * @param int $offset
+     * @param int $limit
+     * @return IComment[]
+     */
+    public function searchForObjects(string $search, string $objectType, array $objectIds, string $verb, int $offset, int $limit = 50): array {
+        $query = $this->dbConn->getQueryBuilder();
+
+        $query->select('*')
+            ->from('comments')
+            ->where($query->expr()->iLike('message', $query->createNamedParameter(
+                '%' . $this->dbConn->escapeLikeParameter($search). '%'
+            )))
+            ->orderBy('creation_timestamp', 'DESC')
+            ->addOrderBy('id', 'DESC')
+            ->setMaxResults($limit);
+
+        if ($objectType !== '') {
+            $query->andWhere($query->expr()->eq('object_type', $query->createNamedParameter($objectType)));
+        }
+        if (!empty($objectIds)) {
+            $query->andWhere($query->expr()->in('object_id', $query->createNamedParameter($objectIds, IQueryBuilder::PARAM_STR_ARRAY)));
+        }
+        if ($verb !== '') {
+            $query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
+        }
+        if ($offset !== 0) {
+            $query->setFirstResult($offset);
+        }
+
+        $comments = [];
+        $result = $query->execute();
+        while ($data = $result->fetch()) {
+            $comment = $this->getCommentFromData($data);
+            $this->cache($comment);
+            $comments[] = $comment;
+        }
+        $result->closeCursor();
+
+        return $comments;
+    }
+
+    /**
+     * @param $objectType string the object type, e.g. 'files'
+     * @param $objectId string the id of the object
+     * @param \DateTime $notOlderThan optional, timestamp of the oldest comments
+     * that may be returned
+     * @param string $verb Limit the verb of the comment - Added in 14.0.0
+     * @return Int
+     * @since 9.0.0
+     */
+    public function getNumberOfCommentsForObject($objectType, $objectId, \DateTime $notOlderThan = null, $verb = '') {
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->select($qb->func()->count('id'))
+            ->from('comments')
+            ->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
+            ->setParameter('type', $objectType)
+            ->setParameter('id', $objectId);
+
+        if (!is_null($notOlderThan)) {
+            $query
+                ->andWhere($qb->expr()->gt('creation_timestamp', $qb->createParameter('notOlderThan')))
+                ->setParameter('notOlderThan', $notOlderThan, 'datetime');
+        }
+
+        if ($verb !== '') {
+            $query->andWhere($qb->expr()->eq('verb', $qb->createNamedParameter($verb)));
+        }
+
+        $resultStatement = $query->execute();
+        $data = $resultStatement->fetch(\PDO::FETCH_NUM);
+        $resultStatement->closeCursor();
+        return (int)$data[0];
+    }
+
+    /**
+     * @param string $objectType the object type, e.g. 'files'
+     * @param string[] $objectIds the id of the object
+     * @param IUser $user
+     * @param string $verb Limit the verb of the comment - Added in 14.0.0
+     * @return array Map with object id => # of unread comments
+     * @psalm-return array<string, int>
+     * @since 21.0.0
+     */
+    public function getNumberOfUnreadCommentsForObjects(string $objectType, array $objectIds, IUser $user, $verb = ''): array {
+        $unreadComments = [];
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select('c.object_id', $query->func()->count('c.id', 'num_comments'))
+            ->from('comments', 'c')
+            ->leftJoin('c', 'comments_read_markers', 'm', $query->expr()->andX(
+                $query->expr()->eq('m.user_id', $query->createNamedParameter($user->getUID())),
+                $query->expr()->eq('c.object_type', 'm.object_type'),
+                $query->expr()->eq('c.object_id', 'm.object_id')
+            ))
+            ->where($query->expr()->eq('c.object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->in('c.object_id', $query->createParameter('ids')))
+            ->andWhere($query->expr()->orX(
+                $query->expr()->gt('c.creation_timestamp', 'm.marker_datetime'),
+                $query->expr()->isNull('m.marker_datetime')
+            ))
+            ->groupBy('c.object_id');
+
+        if ($verb !== '') {
+            $query->andWhere($query->expr()->eq('c.verb', $query->createNamedParameter($verb)));
+        }
+
+        $unreadComments = array_fill_keys($objectIds, 0);
+        foreach (array_chunk($objectIds, 1000) as $chunk) {
+            $query->setParameter('ids', $chunk, IQueryBuilder::PARAM_INT_ARRAY);
+
+            $result = $query->executeQuery();
+            while ($row = $result->fetch()) {
+                $unreadComments[$row['object_id']] = (int) $row['num_comments'];
+            }
+            $result->closeCursor();
+        }
+
+        return $unreadComments;
+    }
+
+    /**
+     * @param string $objectType
+     * @param string $objectId
+     * @param int $lastRead
+     * @param string $verb
+     * @return int
+     * @since 21.0.0
+     */
+    public function getNumberOfCommentsForObjectSinceComment(string $objectType, string $objectId, int $lastRead, string $verb = ''): int {
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select($query->func()->count('id', 'num_messages'))
+            ->from('comments')
+            ->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
+            ->andWhere($query->expr()->gt('id', $query->createNamedParameter($lastRead)));
+
+        if ($verb !== '') {
+            $query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
+        }
+
+        $result = $query->execute();
+        $data = $result->fetch();
+        $result->closeCursor();
+
+        return (int) ($data['num_messages'] ?? 0);
+    }
+
+    /**
+     * @param string $objectType
+     * @param string $objectId
+     * @param \DateTime $beforeDate
+     * @param string $verb
+     * @return int
+     * @since 21.0.0
+     */
+    public function getLastCommentBeforeDate(string $objectType, string $objectId, \DateTime $beforeDate, string $verb = ''): int {
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select('id')
+            ->from('comments')
+            ->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
+            ->andWhere($query->expr()->lt('creation_timestamp', $query->createNamedParameter($beforeDate, IQueryBuilder::PARAM_DATE)))
+            ->orderBy('creation_timestamp', 'desc');
+
+        if ($verb !== '') {
+            $query->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)));
+        }
+
+        $result = $query->execute();
+        $data = $result->fetch();
+        $result->closeCursor();
+
+        return (int) ($data['id'] ?? 0);
+    }
+
+    /**
+     * @param string $objectType
+     * @param string $objectId
+     * @param string $verb
+     * @param string $actorType
+     * @param string[] $actors
+     * @return \DateTime[] Map of "string actor" => "\DateTime most recent comment date"
+     * @psalm-return array<string, \DateTime>
+     * @since 21.0.0
+     */
+    public function getLastCommentDateByActor(
+        string $objectType,
+        string $objectId,
+        string $verb,
+        string $actorType,
+        array $actors
+    ): array {
+        $lastComments = [];
+
+        $query = $this->dbConn->getQueryBuilder();
+        $query->select('actor_id')
+            ->selectAlias($query->createFunction('MAX(' . $query->getColumnName('creation_timestamp') . ')'), 'last_comment')
+            ->from('comments')
+            ->where($query->expr()->eq('object_type', $query->createNamedParameter($objectType)))
+            ->andWhere($query->expr()->eq('object_id', $query->createNamedParameter($objectId)))
+            ->andWhere($query->expr()->eq('verb', $query->createNamedParameter($verb)))
+            ->andWhere($query->expr()->eq('actor_type', $query->createNamedParameter($actorType)))
+            ->andWhere($query->expr()->in('actor_id', $query->createNamedParameter($actors, IQueryBuilder::PARAM_STR_ARRAY)))
+            ->groupBy('actor_id');
+
+        $result = $query->execute();
+        while ($row = $result->fetch()) {
+            $lastComments[$row['actor_id']] = $this->timeFactory->getDateTime($row['last_comment']);
+        }
+        $result->closeCursor();
+
+        return $lastComments;
+    }
+
+    /**
+     * Get the number of unread comments for all files in a folder
+     *
+     * @param int $folderId
+     * @param IUser $user
+     * @return array [$fileId => $unreadCount]
+     */
+    public function getNumberOfUnreadCommentsForFolder($folderId, IUser $user) {
+        $qb = $this->dbConn->getQueryBuilder();
+
+        $query = $qb->select('f.fileid')
+            ->addSelect($qb->func()->count('c.id', 'num_ids'))
+            ->from('filecache', 'f')
+            ->leftJoin('f', 'comments', 'c', $qb->expr()->andX(
+                $qb->expr()->eq('f.fileid', $qb->expr()->castColumn('c.object_id', IQueryBuilder::PARAM_INT)),
+                $qb->expr()->eq('c.object_type', $qb->createNamedParameter('files'))
+            ))
+            ->leftJoin('c', 'comments_read_markers', 'm', $qb->expr()->andX(
+                $qb->expr()->eq('c.object_id', 'm.object_id'),
+                $qb->expr()->eq('m.object_type', $qb->createNamedParameter('files'))
+            ))
+            ->where(
+                $qb->expr()->andX(
+                    $qb->expr()->eq('f.parent', $qb->createNamedParameter($folderId)),
+                    $qb->expr()->orX(
+                        $qb->expr()->eq('c.object_type', $qb->createNamedParameter('files')),
+                        $qb->expr()->isNull('c.object_type')
+                    ),
+                    $qb->expr()->orX(
+                        $qb->expr()->eq('m.object_type', $qb->createNamedParameter('files')),
+                        $qb->expr()->isNull('m.object_type')
+                    ),
+                    $qb->expr()->orX(
+                        $qb->expr()->eq('m.user_id', $qb->createNamedParameter($user->getUID())),
+                        $qb->expr()->isNull('m.user_id')
+                    ),
+                    $qb->expr()->orX(
+                        $qb->expr()->gt('c.creation_timestamp', 'm.marker_datetime'),
+                        $qb->expr()->isNull('m.marker_datetime')
+                    )
+                )
+            )->groupBy('f.fileid');
+
+        $resultStatement = $query->execute();
+
+        $results = [];
+        while ($row = $resultStatement->fetch()) {
+            $results[$row['fileid']] = (int) $row['num_ids'];
+        }
+        $resultStatement->closeCursor();
+        return $results;
+    }
+
+    /**
+     * creates a new comment and returns it. At this point of time, it is not
+     * saved in the used data storage. Use save() after setting other fields
+     * of the comment (e.g. message or verb).
+     *
+     * @param string $actorType the actor type (e.g. 'users')
+     * @param string $actorId a user id
+     * @param string $objectType the object type the comment is attached to
+     * @param string $objectId the object id the comment is attached to
+     * @return IComment
+     * @since 9.0.0
+     */
+    public function create($actorType, $actorId, $objectType, $objectId) {
+        $comment = new Comment();
+        $comment
+            ->setActor($actorType, $actorId)
+            ->setObject($objectType, $objectId);
+        return $comment;
+    }
+
+    /**
+     * permanently deletes the comment specified by the ID
+     *
+     * When the comment has child comments, their parent ID will be changed to
+     * the parent ID of the item that is to be deleted.
+     *
+     * @param string $id
+     * @return bool
+     * @throws \InvalidArgumentException
+     * @since 9.0.0
+     */
+    public function delete($id) {
+        if (!is_string($id)) {
+            throw new \InvalidArgumentException('Parameter must be string');
+        }
+
+        try {
+            $comment = $this->get($id);
+        } catch (\Exception $e) {
+            // Ignore exceptions, we just don't fire a hook then
+            $comment = null;
+        }
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->delete('comments')
+            ->where($qb->expr()->eq('id', $qb->createParameter('id')))
+            ->setParameter('id', $id);
+
+        try {
+            $affectedRows = $query->execute();
+            $this->uncache($id);
+        } catch (DriverException $e) {
+            $this->logger->error($e->getMessage(), [
+                'exception' => $e,
+                'app' => 'core_comments',
+            ]);
+            return false;
+        }
+
+        if ($affectedRows > 0 && $comment instanceof IComment) {
+            $this->sendEvent(CommentsEvent::EVENT_DELETE, $comment);
+        }
+
+        return ($affectedRows > 0);
+    }
+
+    /**
+     * saves the comment permanently
+     *
+     * if the supplied comment has an empty ID, a new entry comment will be
+     * saved and the instance updated with the new ID.
+     *
+     * Otherwise, an existing comment will be updated.
+     *
+     * Throws NotFoundException when a comment that is to be updated does not
+     * exist anymore at this point of time.
+     *
+     * @param IComment $comment
+     * @return bool
+     * @throws NotFoundException
+     * @since 9.0.0
+     */
+    public function save(IComment $comment) {
+        if ($this->prepareCommentForDatabaseWrite($comment)->getId() === '') {
+            $result = $this->insert($comment);
+        } else {
+            $result = $this->update($comment);
+        }
+
+        if ($result && !!$comment->getParentId()) {
+            $this->updateChildrenInformation(
+                $comment->getParentId(),
+                $comment->getCreationDateTime()
+            );
+            $this->cache($comment);
+        }
+
+        return $result;
+    }
+
+    /**
+     * inserts the provided comment in the database
+     *
+     * @param IComment $comment
+     * @return bool
+     */
+    protected function insert(IComment $comment): bool {
+        try {
+            $result = $this->insertQuery($comment, true);
+        } catch (InvalidFieldNameException $e) {
+            // The reference id field was only added in Nextcloud 19.
+            // In order to not cause too long waiting times on the update,
+            // it was decided to only add it lazy, as it is also not a critical
+            // feature, but only helps to have a better experience while commenting.
+            // So in case the reference_id field is missing,
+            // we simply save the comment without that field.
+            $result = $this->insertQuery($comment, false);
+        }
+
+        return $result;
+    }
+
+    protected function insertQuery(IComment $comment, bool $tryWritingReferenceId): bool {
+        $qb = $this->dbConn->getQueryBuilder();
+
+        $values = [
+            'parent_id' => $qb->createNamedParameter($comment->getParentId()),
+            'topmost_parent_id' => $qb->createNamedParameter($comment->getTopmostParentId()),
+            'children_count' => $qb->createNamedParameter($comment->getChildrenCount()),
+            'actor_type' => $qb->createNamedParameter($comment->getActorType()),
+            'actor_id' => $qb->createNamedParameter($comment->getActorId()),
+            'message' => $qb->createNamedParameter($comment->getMessage()),
+            'verb' => $qb->createNamedParameter($comment->getVerb()),
+            'creation_timestamp' => $qb->createNamedParameter($comment->getCreationDateTime(), 'datetime'),
+            'latest_child_timestamp' => $qb->createNamedParameter($comment->getLatestChildDateTime(), 'datetime'),
+            'object_type' => $qb->createNamedParameter($comment->getObjectType()),
+            'object_id' => $qb->createNamedParameter($comment->getObjectId()),
+        ];
+
+        if ($tryWritingReferenceId) {
+            $values['reference_id'] = $qb->createNamedParameter($comment->getReferenceId());
+        }
+
+        $affectedRows = $qb->insert('comments')
+            ->values($values)
+            ->execute();
+
+        if ($affectedRows > 0) {
+            $comment->setId((string)$qb->getLastInsertId());
+            $this->sendEvent(CommentsEvent::EVENT_ADD, $comment);
+        }
+
+        return $affectedRows > 0;
+    }
+
+    /**
+     * updates a Comment data row
+     *
+     * @param IComment $comment
+     * @return bool
+     * @throws NotFoundException
+     */
+    protected function update(IComment $comment) {
+        // for properly working preUpdate Events we need the old comments as is
+        // in the DB and overcome caching. Also avoid that outdated information stays.
+        $this->uncache($comment->getId());
+        $this->sendEvent(CommentsEvent::EVENT_PRE_UPDATE, $this->get($comment->getId()));
+        $this->uncache($comment->getId());
+
+        try {
+            $result = $this->updateQuery($comment, true);
+        } catch (InvalidFieldNameException $e) {
+            // See function insert() for explanation
+            $result = $this->updateQuery($comment, false);
+        }
+
+        $this->sendEvent(CommentsEvent::EVENT_UPDATE, $comment);
+
+        return $result;
+    }
+
+    protected function updateQuery(IComment $comment, bool $tryWritingReferenceId): bool {
+        $qb = $this->dbConn->getQueryBuilder();
+        $qb
+            ->update('comments')
+            ->set('parent_id', $qb->createNamedParameter($comment->getParentId()))
+            ->set('topmost_parent_id', $qb->createNamedParameter($comment->getTopmostParentId()))
+            ->set('children_count', $qb->createNamedParameter($comment->getChildrenCount()))
+            ->set('actor_type', $qb->createNamedParameter($comment->getActorType()))
+            ->set('actor_id', $qb->createNamedParameter($comment->getActorId()))
+            ->set('message', $qb->createNamedParameter($comment->getMessage()))
+            ->set('verb', $qb->createNamedParameter($comment->getVerb()))
+            ->set('creation_timestamp', $qb->createNamedParameter($comment->getCreationDateTime(), 'datetime'))
+            ->set('latest_child_timestamp', $qb->createNamedParameter($comment->getLatestChildDateTime(), 'datetime'))
+            ->set('object_type', $qb->createNamedParameter($comment->getObjectType()))
+            ->set('object_id', $qb->createNamedParameter($comment->getObjectId()));
+
+        if ($tryWritingReferenceId) {
+            $qb->set('reference_id', $qb->createNamedParameter($comment->getReferenceId()));
+        }
+
+        $affectedRows = $qb->where($qb->expr()->eq('id', $qb->createNamedParameter($comment->getId())))
+            ->execute();
+
+        if ($affectedRows === 0) {
+            throw new NotFoundException('Comment to update does ceased to exist');
+        }
+
+        return $affectedRows > 0;
+    }
+
+    /**
+     * removes references to specific actor (e.g. on user delete) of a comment.
+     * The comment itself must not get lost/deleted.
+     *
+     * @param string $actorType the actor type (e.g. 'users')
+     * @param string $actorId a user id
+     * @return boolean
+     * @since 9.0.0
+     */
+    public function deleteReferencesOfActor($actorType, $actorId) {
+        $this->checkRoleParameters('Actor', $actorType, $actorId);
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $affectedRows = $qb
+            ->update('comments')
+            ->set('actor_type', $qb->createNamedParameter(ICommentsManager::DELETED_USER))
+            ->set('actor_id', $qb->createNamedParameter(ICommentsManager::DELETED_USER))
+            ->where($qb->expr()->eq('actor_type', $qb->createParameter('type')))
+            ->andWhere($qb->expr()->eq('actor_id', $qb->createParameter('id')))
+            ->setParameter('type', $actorType)
+            ->setParameter('id', $actorId)
+            ->execute();
+
+        $this->commentsCache = [];
+
+        return is_int($affectedRows);
+    }
+
+    /**
+     * deletes all comments made of a specific object (e.g. on file delete)
+     *
+     * @param string $objectType the object type (e.g. 'files')
+     * @param string $objectId e.g. the file id
+     * @return boolean
+     * @since 9.0.0
+     */
+    public function deleteCommentsAtObject($objectType, $objectId) {
+        $this->checkRoleParameters('Object', $objectType, $objectId);
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $affectedRows = $qb
+            ->delete('comments')
+            ->where($qb->expr()->eq('object_type', $qb->createParameter('type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('id')))
+            ->setParameter('type', $objectType)
+            ->setParameter('id', $objectId)
+            ->execute();
+
+        $this->commentsCache = [];
+
+        return is_int($affectedRows);
+    }
+
+    /**
+     * deletes the read markers for the specified user
+     *
+     * @param \OCP\IUser $user
+     * @return bool
+     * @since 9.0.0
+     */
+    public function deleteReadMarksFromUser(IUser $user) {
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->delete('comments_read_markers')
+            ->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
+            ->setParameter('user_id', $user->getUID());
+
+        try {
+            $affectedRows = $query->execute();
+        } catch (DriverException $e) {
+            $this->logger->error($e->getMessage(), [
+                'exception' => $e,
+                'app' => 'core_comments',
+            ]);
+            return false;
+        }
+        return ($affectedRows > 0);
+    }
+
+    /**
+     * sets the read marker for a given file to the specified date for the
+     * provided user
+     *
+     * @param string $objectType
+     * @param string $objectId
+     * @param \DateTime $dateTime
+     * @param IUser $user
+     * @since 9.0.0
+     */
+    public function setReadMark($objectType, $objectId, \DateTime $dateTime, IUser $user) {
+        $this->checkRoleParameters('Object', $objectType, $objectId);
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $values = [
+            'user_id' => $qb->createNamedParameter($user->getUID()),
+            'marker_datetime' => $qb->createNamedParameter($dateTime, 'datetime'),
+            'object_type' => $qb->createNamedParameter($objectType),
+            'object_id' => $qb->createNamedParameter($objectId),
+        ];
+
+        // Strategy: try to update, if this does not return affected rows, do an insert.
+        $affectedRows = $qb
+            ->update('comments_read_markers')
+            ->set('user_id', $values['user_id'])
+            ->set('marker_datetime', $values['marker_datetime'])
+            ->set('object_type', $values['object_type'])
+            ->set('object_id', $values['object_id'])
+            ->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
+            ->andWhere($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
+            ->setParameter('user_id', $user->getUID(), IQueryBuilder::PARAM_STR)
+            ->setParameter('object_type', $objectType, IQueryBuilder::PARAM_STR)
+            ->setParameter('object_id', $objectId, IQueryBuilder::PARAM_STR)
+            ->execute();
+
+        if ($affectedRows > 0) {
+            return;
+        }
+
+        $qb->insert('comments_read_markers')
+            ->values($values)
+            ->execute();
+    }
+
+    /**
+     * returns the read marker for a given file to the specified date for the
+     * provided user. It returns null, when the marker is not present, i.e.
+     * no comments were marked as read.
+     *
+     * @param string $objectType
+     * @param string $objectId
+     * @param IUser $user
+     * @return \DateTime|null
+     * @since 9.0.0
+     */
+    public function getReadMark($objectType, $objectId, IUser $user) {
+        $qb = $this->dbConn->getQueryBuilder();
+        $resultStatement = $qb->select('marker_datetime')
+            ->from('comments_read_markers')
+            ->where($qb->expr()->eq('user_id', $qb->createParameter('user_id')))
+            ->andWhere($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
+            ->setParameter('user_id', $user->getUID(), IQueryBuilder::PARAM_STR)
+            ->setParameter('object_type', $objectType, IQueryBuilder::PARAM_STR)
+            ->setParameter('object_id', $objectId, IQueryBuilder::PARAM_STR)
+            ->execute();
+
+        $data = $resultStatement->fetch();
+        $resultStatement->closeCursor();
+        if (!$data || is_null($data['marker_datetime'])) {
+            return null;
+        }
+
+        return new \DateTime($data['marker_datetime']);
+    }
+
+    /**
+     * deletes the read markers on the specified object
+     *
+     * @param string $objectType
+     * @param string $objectId
+     * @return bool
+     * @since 9.0.0
+     */
+    public function deleteReadMarksOnObject($objectType, $objectId) {
+        $this->checkRoleParameters('Object', $objectType, $objectId);
+
+        $qb = $this->dbConn->getQueryBuilder();
+        $query = $qb->delete('comments_read_markers')
+            ->where($qb->expr()->eq('object_type', $qb->createParameter('object_type')))
+            ->andWhere($qb->expr()->eq('object_id', $qb->createParameter('object_id')))
+            ->setParameter('object_type', $objectType)
+            ->setParameter('object_id', $objectId);
+
+        try {
+            $affectedRows = $query->execute();
+        } catch (DriverException $e) {
+            $this->logger->error($e->getMessage(), [
+                'exception' => $e,
+                'app' => 'core_comments',
+            ]);
+            return false;
+        }
+        return ($affectedRows > 0);
+    }
+
+    /**
+     * registers an Entity to the manager, so event notifications can be send
+     * to consumers of the comments infrastructure
+     *
+     * @param \Closure $closure
+     */
+    public function registerEventHandler(\Closure $closure) {
+        $this->eventHandlerClosures[] = $closure;
+        $this->eventHandlers = [];
+    }
+
+    /**
+     * registers a method that resolves an ID to a display name for a given type
+     *
+     * @param string $type
+     * @param \Closure $closure
+     * @throws \OutOfBoundsException
+     * @since 11.0.0
+     *
+     * Only one resolver shall be registered per type. Otherwise a
+     * \OutOfBoundsException has to thrown.
+     */
+    public function registerDisplayNameResolver($type, \Closure $closure) {
+        if (!is_string($type)) {
+            throw new \InvalidArgumentException('String expected.');
+        }
+        if (isset($this->displayNameResolvers[$type])) {
+            throw new \OutOfBoundsException('Displayname resolver for this type already registered');
+        }
+        $this->displayNameResolvers[$type] = $closure;
+    }
+
+    /**
+     * resolves a given ID of a given Type to a display name.
+     *
+     * @param string $type
+     * @param string $id
+     * @return string
+     * @throws \OutOfBoundsException
+     * @since 11.0.0
+     *
+     * If a provided type was not registered, an \OutOfBoundsException shall
+     * be thrown. It is upon the resolver discretion what to return of the
+     * provided ID is unknown. It must be ensured that a string is returned.
+     */
+    public function resolveDisplayName($type, $id) {
+        if (!is_string($type)) {
+            throw new \InvalidArgumentException('String expected.');
+        }
+        if (!isset($this->displayNameResolvers[$type])) {
+            throw new \OutOfBoundsException('No Displayname resolver for this type registered');
+        }
+        return (string)$this->displayNameResolvers[$type]($id);
+    }
+
+    /**
+     * returns valid, registered entities
+     *
+     * @return \OCP\Comments\ICommentsEventHandler[]
+     */
+    private function getEventHandlers() {
+        if (!empty($this->eventHandlers)) {
+            return $this->eventHandlers;
+        }
+
+        $this->eventHandlers = [];
+        foreach ($this->eventHandlerClosures as $name => $closure) {
+            $entity = $closure();
+            if (!($entity instanceof ICommentsEventHandler)) {
+                throw new \InvalidArgumentException('The given entity does not implement the ICommentsEntity interface');
+            }
+            $this->eventHandlers[$name] = $entity;
+        }
+
+        return $this->eventHandlers;
+    }
+
+    /**
+     * sends notifications to the registered entities
+     *
+     * @param $eventType
+     * @param IComment $comment
+     */
+    private function sendEvent($eventType, IComment $comment) {
+        $entities = $this->getEventHandlers();
+        $event = new CommentsEvent($eventType, $comment);
+        foreach ($entities as $entity) {
+            $entity->handle($event);
+        }
+    }
+
+    /**
+     * Load the Comments app into the page
+     *
+     * @since 21.0.0
+     */
+    public function load(): void {
+        $this->initialStateService->provideInitialState('comments', 'max-message-length', IComment::MAX_MESSAGE_LENGTH);
+        Util::addScript('comments', 'comments-app');
+    }
 }