add icommentsmanger and icomment implementation

register CommentsManager service, allow override, document in config.sample.php

don't insert autoincrement ids in tests, because of dislikes from oracle and pgsql

specify timezone in null date

only accepts strings for ID parameter that can be converted to int

replace forgotten hardcoded IDs in tests

react on deleted users

react on file deletion

Postgresql compatibility

lastInsertId needs *PREFIX* with the table name

do not listen for file deletion, because it is not reliable (trashbin, external storages)

add runtime cache for comments

12 files changed, 1706 insertions, 6 deletions

diff --git a/config/config.sample.php b/config/config.sample.php
index c3abe3a2b87..9b10792944f 100644
--- a/config/config.sample.php
+++ b/config/config.sample.php
@@ -815,6 +815,19 @@ $CONFIG = array(
 'enforce_home_folder_naming_rule' => true,
 
 /**
+ * Comments
+ *
+ * Global settings for the Comments infrastructure
+ */
+
+/**
+ * Replaces the default Comments Manager Factory. This can be utilized if an
+ * own or 3rdParty CommentsManager should be used that – for instance – uses the
+ * filesystem instead of the database to keep the comments.
+ */
+'comments.managerFactory' => '\OC\Comments\ManagerFactory',
+
+/**
  * Maintenance
  *
  * These options are for halting user activity when you are performing server
diff --git a/lib/private/comments/comment.php b/lib/private/comments/comment.php
new file mode 100644
index 00000000000..2e9e4bd3d84
--- /dev/null
+++ b/lib/private/comments/comment.php
@@ -0,0 +1,350 @@
+<?php
+
+namespace OC\Comments;
+
+use OCP\Comments\IComment;
+use OCP\Comments\IllegalIDChangeException;
+
+class Comment implements IComment {
+
+	protected $data = [
+		'id'              => '',
+		'parentId'        => '0',
+		'topmostParentId' => '0',
+		'childrenCount'   => '0',
+		'message'         => '',
+		'verb'            => '',
+		'actorType'       => '',
+		'actorId'         => '',
+		'objectType'      => '',
+		'objectId'        => '',
+		'creationDT'      => null,
+		'latestChildDT'   => null,
+	];
+
+	/**
+	 * Comment constructor.
+	 *
+	 * @param [] $data	optional, array with keys according to column names from
+	 * 					the comments database scheme
+	 */
+	public function __construct(array $data = null) {
+		if(is_array($data)) {
+			$this->fromArray($data);
+		}
+	}
+
+	/**
+	 * returns the ID of the comment
+	 *
+	 * It may return an empty string, if the comment was not stored.
+	 * It is expected that the concrete Comment implementation gives an ID
+	 * by itself (e.g. after saving).
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getId() {
+		return $this->data['id'];
+	}
+
+	/**
+	 * sets the ID of the comment and returns itself
+	 *
+	 * It is only allowed to set the ID only, if the current id is an empty
+	 * string (which means it is not stored in a database, storage or whatever
+	 * the concrete implementation does), or vice versa. Changing a given ID is
+	 * not permitted and must result in an IllegalIDChangeException.
+	 *
+	 * @param string $id
+	 * @return IComment
+	 * @throws IllegalIDChangeException
+	 * @since 9.0.0
+	 */
+	public function setId($id) {
+		if(!is_string($id)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+
+		if($this->data['id'] === '' || ($this->data['id'] !== '' && $id === '')) {
+			$this->data['id'] = $id;
+			return $this;
+		}
+
+		throw new IllegalIDChangeException('Not allowed to assign a new ID to an already saved comment.');
+	}
+
+	/**
+	 * returns the parent ID of the comment
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getParentId() {
+		return $this->data['parentId'];
+	}
+
+	/**
+	 * sets the parent ID and returns itself
+	 *
+	 * @param string $parentId
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setParentId($parentId) {
+		if(!is_string($parentId)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['parentId'] = $parentId;
+		return $this;
+	}
+
+	/**
+	 * returns the topmost parent ID of the comment
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getTopmostParentId() {
+		return $this->data['topmostParentId'];
+	}
+
+
+	/**
+	 * sets the topmost parent ID and returns itself
+	 *
+	 * @param string $id
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setTopmostParentId($id) {
+		if(!is_string($id)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['topmostParentId'] = $id;
+		return $this;
+	}
+
+	/**
+	 * returns the number of children
+	 *
+	 * @return int
+	 * @since 9.0.0
+	 */
+	public function getChildrenCount() {
+		return $this->data['childrenCount'];
+	}
+
+	/**
+	 * sets the number of children
+	 *
+	 * @param int $count
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setChildrenCount($count) {
+		if(!is_int($count)) {
+			throw new \InvalidArgumentException('Integer expected.');
+		}
+		$this->data['childrenCount'] = $count;
+		return $this;
+	}
+
+	/**
+	 * returns the message of the comment
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getMessage() {
+		return $this->data['message'];
+	}
+
+	/**
+	 * sets the message of the comment and returns itself
+	 *
+	 * @param string $message
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setMessage($message) {
+		if(!is_string($message)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['message'] = $message;
+		return $this;
+	}
+
+	/**
+	 * returns the verb of the comment
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getVerb() {
+		return $this->data['verb'];
+	}
+
+	/**
+	 * sets the verb of the comment, e.g. 'comment' or 'like'
+	 *
+	 * @param string $verb
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setVerb($verb) {
+		if(!is_string($verb)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['verb'] = $verb;
+		return $this;
+	}
+
+	/**
+	 * returns the actor type
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getActorType() {
+		return $this->data['actorType'];
+	}
+
+	/**
+	 * returns the actor ID
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getActorId() {
+		return $this->data['actorId'];
+	}
+
+	/**
+	 * sets (overwrites) the actor type and id
+	 *
+	 * @param string $actorType e.g. 'user'
+	 * @param string $actorId e.g. 'zombie234'
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setActor($actorType, $actorId) {
+		if(!is_string($actorType) || !is_string($actorId)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['actorType'] = $actorType;
+		$this->data['actorId']   = $actorId;
+		return $this;
+	}
+
+	/**
+	 * returns the creation date of the comment.
+	 *
+	 * If not explicitly set, it shall default to the time of initialization.
+	 *
+	 * @return \DateTime
+	 * @since 9.0.0
+	 */
+	public function getCreationDateTime() {
+		return $this->data['creationDT'];
+	}
+
+	/**
+	 * sets the creation date of the comment and returns itself
+	 *
+	 * @param \DateTime $timestamp
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setCreationDateTime(\DateTime $timestamp) {
+		$this->data['creationDT'] = $timestamp;
+		return $this;
+	}
+
+	/**
+	 * returns the timestamp of the most recent child
+	 *
+	 * @return int
+	 * @since 9.0.0
+	 */
+	public function getLatestChildDateTime() {
+		return $this->data['latestChildDT'];
+	}
+
+	/**
+	 * sets the date of the most recent child
+	 *
+	 * @param \DateTime $dateTime
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setLatestChildDateTime(\DateTime $dateTime = null) {
+		$this->data['latestChildDT'] = $dateTime;
+		return $this;
+	}
+
+	/**
+	 * returns the object type the comment is attached to
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getObjectType() {
+		return $this->data['objectType'];
+	}
+
+	/**
+	 * returns the object id the comment is attached to
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getObjectId() {
+		return $this->data['objectId'];
+	}
+
+	/**
+	 * sets (overwrites) the object of the comment
+	 *
+	 * @param string $objectType e.g. 'file'
+	 * @param string $objectId e.g. '16435'
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setObject($objectType, $objectId) {
+		if(!is_string($objectType) || !is_string($objectId)) {
+			throw new \InvalidArgumentException('String expected.');
+		}
+		$this->data['objectType'] = $objectType;
+		$this->data['objectId']   = $objectId;
+		return $this;
+	}
+
+	/**
+	 * sets the comment data based on an array with keys as taken from the
+	 * database.
+	 *
+	 * @param [] $data
+	 * @return IComment
+	 */
+	protected function fromArray($data) {
+		foreach(array_keys($data) as $key) {
+			// translate DB keys to internal setter names
+			$setter = 'set' . str_replace('_', '', ucwords($key,'_'));
+			$setter = str_replace('Timestamp', 'DateTime', $setter);
+
+			if(method_exists($this, $setter)) {
+				$this->$setter($data[$key]);
+			}
+		}
+
+		foreach(['actor', 'object'] as $role) {
+			if(isset($data[$role . '_type']) && isset($data[$role . '_id'])) {
+				$setter = 'set' . ucfirst($role);
+				$this->$setter($data[$role . '_type'], $data[$role . '_id']);
+			}
+		}
+
+		return $this;
+	}
+}
diff --git a/lib/private/comments/manager.php b/lib/private/comments/manager.php
new file mode 100644
index 00000000000..78b2b71c8dd
--- /dev/null
+++ b/lib/private/comments/manager.php
@@ -0,0 +1,566 @@
+<?php
+
+namespace OC\Comments;
+
+use Doctrine\DBAL\Exception\DriverException;
+use OC\Hooks\Emitter;
+use OCP\Comments\IComment;
+use OCP\Comments\ICommentsManager;
+use OCP\Comments\NotFoundException;
+use OCP\IDBConnection;
+use OCP\ILogger;
+
+class Manager implements ICommentsManager {
+
+	/** @var  IDBConnection */
+	protected $dbConn;
+
+	/** @var  ILogger */
+	protected $logger;
+
+	/** @var IComment[]  */
+	protected $commentsCache = [];
+
+	public function __construct(
+		IDBConnection $dbConn,
+		Emitter $userManager,
+		ILogger $logger
+	) {
+		$this->dbConn = $dbConn;
+		$this->logger = $logger;
+		$userManager->listen('\OC\User', 'postDelete', function($user) {
+			/** @var \OCP\IUser $user */
+			$this->deleteReferencesOfActor('user', $user->getUid());
+		});
+	}
+
+	/**
+	 * converts data base data into PHP native, proper types as defined by
+	 * IComment interface.
+	 *
+	 * @param array $data
+	 * @return array
+	 */
+	protected function normalizeDatabaseData(array $data) {
+		$data['id'] = strval($data['id']);
+		$data['parent_id'] = strval($data['parent_id']);
+		$data['topmost_parent_id'] = strval($data['topmost_parent_id']);
+		$data['creation_timestamp'] = new \DateTime($data['creation_timestamp']);
+		$data['latest_child_timestamp'] = new \DateTime($data['latest_child_timestamp']);
+		$data['children_count'] = intval($data['children_count']);
+		return $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
+	 * @throws \UnexpectedValueException
+	 */
+	protected function prepareCommentForDatabaseWrite(IComment $comment) {
+		if(    empty($comment->getActorType())
+			|| empty($comment->getActorId())
+			|| empty($comment->getObjectType())
+			|| empty($comment->getObjectId())
+			|| empty($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();
+		} else {
+			return $this->determineTopmostParentId($comment->getId());
+		}
+	}
+
+	/**
+	 * 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->createFunction('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();
+		$children = intval($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;
+		}
+		$this->commentsCache[strval($id)] = $comment;
+	}
+
+	/**
+	 * removes an entry from the comments run time cache
+	 *
+	 * @param mixed $id the comment's id
+	 */
+	protected function uncache($id) {
+		$id = strval($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(intval($id) === 0) {
+			throw new \InvalidArgumentException('IDs must be translatable to a number in this implementation.');
+		}
+
+		if(isset($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, \PDO::PARAM_INT)
+			->execute();
+
+		$data = $resultStatement->fetch();
+		$resultStatement->closeCursor();
+		if(!$data) {
+			throw new NotFoundException();
+		}
+
+		$comment = new Comment($this->normalizeDatabaseData($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()) {
+			$comment = new Comment($this->normalizeDatabaseData($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()) {
+			$comment = new Comment($this->normalizeDatabaseData($data));
+			$this->cache($comment);
+			$comments[] = $comment;
+		}
+		$resultStatement->closeCursor();
+
+		return $comments;
+	}
+
+	/**
+	 * @param $objectType string the object type, e.g. 'files'
+	 * @param $objectId string the id of the object
+	 * @return Int
+	 * @since 9.0.0
+	 */
+	public function getNumberOfCommentsForObject($objectType, $objectId) {
+		$qb = $this->dbConn->getQueryBuilder();
+		$query = $qb->select($qb->createFunction('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);
+
+		$resultStatement = $query->execute();
+		$data = $resultStatement->fetch(\PDO::FETCH_NUM);
+		$resultStatement->closeCursor();
+		return intval($data[0]);
+	}
+
+	/**
+	 * 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. 'user')
+	 * @param string $actorId a user id
+	 * @param string $objectType the object type the comment is attached to
+	 * @param string $objectId the object id the comment is attached to
+	 * @return IComment
+	 * @throws \InvalidArgumentException
+	 * @since 9.0.0
+	 */
+	public function create($actorType, $actorId, $objectType, $objectId) {
+		if(
+			   !is_string($actorType)  || empty($actorType)
+			|| !is_string($actorId)    || empty($actorId)
+		    || !is_string($objectType) || empty($objectType)
+		    || !is_string($objectId)   || empty($objectId)
+		) {
+			// unsure whether it's a good place to enforce it here, since the
+			// comment instance can be manipulated anyway.
+			throw new \InvalidArgumentException('All arguments must be non-empty strings');
+		}
+		$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');
+		}
+
+		$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) {
+			$this->logger->logException($e, ['app' => 'core_comments']);
+			return false;
+		}
+		return ($affectedRows > 0);
+	}
+
+	/**
+	 * saves the comment permanently and returns it
+	 *
+	 * if the supplied comment has an empty ID, a new entry comment will be
+	 * saved and the instance updated with the new ID.
+	 *
+	 * Otherwise, an existing comment will be updated.
+	 *
+	 * Throws NotFoundException when a comment that is to be updated does not
+	 * exist anymore at this point of time.
+	 *
+	 * @param IComment &$comment
+	 * @return bool
+	 * @throws NotFoundException
+	 * @since 9.0.0
+	 */
+	public function save(IComment &$comment) {
+		$comment = $this->prepareCommentForDatabaseWrite($comment);
+		if($comment->getId() === '') {
+			$result = $this->insert($comment);
+		} else {
+			$result = $this->update($comment);
+		}
+
+		if($result && !empty($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) {
+		$qb = $this->dbConn->getQueryBuilder();
+		$affectedRows = $qb
+			->insert('comments')
+			->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()),
+			])
+			->execute();
+
+		if ($affectedRows > 0) {
+			$comment->setId(strval($this->dbConn->lastInsertId('*PREFIX*comments')));
+		}
+
+		return $affectedRows > 0;
+	}
+
+	/**
+	 * updates a Comment data row
+	 *
+	 * @param IComment $comment
+	 * @return bool
+	 * @throws NotFoundException
+	 */
+	protected function update(IComment $comment) {
+		$qb = $this->dbConn->getQueryBuilder();
+		$affectedRows = $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()))
+			->where($qb->expr()->eq('id', $qb->createParameter('id')))
+			->setParameter('id', $comment->getId())
+			->execute();
+
+		if($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. 'user')
+	 * @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. 'file')
+	 * @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);
+	}
+}
diff --git a/lib/private/comments/managerfactory.php b/lib/private/comments/managerfactory.php
new file mode 100644
index 00000000000..71d73571b10
--- /dev/null
+++ b/lib/private/comments/managerfactory.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace OC\Comments;
+
+use OCP\Comments\ICommentsManager;
+use OCP\Comments\ICommentsManagerFactory;
+
+
+class ManagerFactory implements ICommentsManagerFactory {
+
+	/**
+	 * creates and returns an instance of the ICommentsManager
+	 *
+	 * @return ICommentsManager
+	 * @since 9.0.0
+	 */
+	public function getManager() {
+		return new Manager(
+			\oc::$server->getDatabaseConnection(),
+			\oc::$server->getUserManager(),
+			\oc::$server->getLogger()
+		);
+	}
+}
diff --git a/lib/private/server.php b/lib/private/server.php
index 6692e6f6bbf..ecac18d6a9b 100644
--- a/lib/private/server.php
+++ b/lib/private/server.php
@@ -528,6 +528,13 @@ class Server extends SimpleContainer implements IServerContainer {
 			});
 			return $manager;
 		});
+		$this->registerService('CommentsManager', function(Server $c) {
+			$config = $c->getConfig();
+			$factoryClass = $config->getSystemValue('comments.managerFactory', '\OC\Comments\ManagerFactory');
+			/** @var \OCP\Comments\ICommentsManagerFactory $factory */
+			$factory = new $factoryClass();
+			return $factory->getManager();
+		});
 		$this->registerService('EventDispatcher', function() {
 			return new EventDispatcher();
 		});
@@ -1121,6 +1128,10 @@ class Server extends SimpleContainer implements IServerContainer {
 		return $this->query('NotificationManager');
 	}
 
+	public function getCommentsManager() {
+		return $this->query('CommentsManager');
+	}
+
 	/**
 	 * @return \OC\IntegrityCheck\Checker
 	 */
diff --git a/lib/public/comments/icomment.php b/lib/public/comments/icomment.php
index c8f407624a0..8cfc504fc3f 100644
--- a/lib/public/comments/icomment.php
+++ b/lib/public/comments/icomment.php
@@ -49,7 +49,6 @@ interface IComment {
 
 	/**
 	 * sets the parent ID and returns itself
-	 *
 	 * @param string $parentId
 	 * @return IComment
 	 * @since 9.0.0
@@ -57,6 +56,24 @@ interface IComment {
 	public function setParentId($parentId);
 
 	/**
+	 * returns the topmost parent ID of the comment
+	 *
+	 * @return string
+	 * @since 9.0.0
+	 */
+	public function getTopmostParentId();
+
+
+	/**
+	 * sets the topmost parent ID and returns itself
+	 *
+	 * @param string $id
+	 * @return IComment
+	 * @since 9.0.0
+	 */
+	public function setTopmostParentId($id);
+
+	/**
 	 * returns the number of children
 	 *
 	 * @return int
diff --git a/lib/public/comments/icommentsmanager.php b/lib/public/comments/icommentsmanager.php
index ebf7a34bf27..f6883224d60 100644
--- a/lib/public/comments/icommentsmanager.php
+++ b/lib/public/comments/icommentsmanager.php
@@ -13,6 +13,17 @@ namespace OCP\Comments;
 interface ICommentsManager {
 
 	/**
+	 * @const DELETED_USER type and id for a user that has been deleted
+	 * @see deleteReferencesOfActor
+	 * @since 9.0.0
+	 *
+	 * To be used as replacement for user type actors in deleteReferencesOfActor().
+	 *
+	 * User interfaces shall show "Deleted user" as display name, if needed.
+	 */
+	const DELETED_USER = 'deleted_user';
+
+	/**
 	 * returns a comment instance
 	 *
 	 * @param string $id the ID of the comment
@@ -28,7 +39,7 @@ interface ICommentsManager {
 	 * @param string $id
 	 * @param int $limit max number of entries to return, 0 returns all
 	 * @param int $offset the start entry
-	 * @return []
+	 * @return array
 	 * @since 9.0.0
 	 *
 	 * The return array looks like this
@@ -73,7 +84,6 @@ interface ICommentsManager {
 	 * @param \DateTime $notOlderThan optional, timestamp of the oldest comments
 	 * that may be returned
 	 * @return IComment[]
-	 * @throws NotFoundException in case the requested type or id is not present
 	 * @since 9.0.0
 	 */
 	public function getForObject(
@@ -88,7 +98,6 @@ interface ICommentsManager {
 	 * @param $objectType string the object type, e.g. 'files'
 	 * @param $objectId string the id of the object
 	 * @return Int
-	 * @throws NotFoundException in case the requested type or id is not present
 	 * @since 9.0.0
 	 */
 	public function getNumberOfCommentsForObject($objectType, $objectId);
@@ -130,17 +139,20 @@ interface ICommentsManager {
 	 * Throws NotFoundException when a comment that is to be updated does not
 	 * exist anymore at this point of time.
 	 *
-	 * @param IComment
+	 * @param IComment &$comment
 	 * @return bool
 	 * @throws NotFoundException
 	 * @since 9.0.0
 	 */
-	public function save(&$comment);
+	public function save(IComment &$comment);
 
 	/**
 	 * removes references to specific actor (e.g. on user delete) of a comment.
 	 * The comment itself must not get lost/deleted.
 	 *
+	 * A 'user' type actor (type and id) should get replaced by the
+	 * value of the DELETED_USER constant of this interface.
+	 *
 	 * @param string $actorType the actor type (e.g. 'user')
 	 * @param string $actorId a user id
 	 * @return boolean
diff --git a/lib/public/comments/icommentsmanagerfactory.php b/lib/public/comments/icommentsmanagerfactory.php
new file mode 100644
index 00000000000..7bfb79aae00
--- /dev/null
+++ b/lib/public/comments/icommentsmanagerfactory.php
@@ -0,0 +1,22 @@
+<?php
+
+namespace OCP\Comments;
+
+/**
+ * Interface IComment
+ *
+ * This class represents a comment and offers methods for modification.
+ *
+ * @package OCP\Comments
+ * @since 9.0.0
+ */
+interface ICommentsManagerFactory {
+
+	/**
+	 * creates and returns an instance of the ICommentsManager
+	 *
+	 * @return ICommentsManager
+	 * @since 9.0.0
+	 */
+	public function getManager();
+}
diff --git a/tests/lib/comments/comment.php b/tests/lib/comments/comment.php
new file mode 100644
index 00000000000..790aca42967
--- /dev/null
+++ b/tests/lib/comments/comment.php
@@ -0,0 +1,102 @@
+<?php
+
+class Test_Comments_Comment extends Test\TestCase
+{
+
+	public function testSettersValidInput() {
+		$comment = new \OC\Comments\Comment();
+
+		$id = 'comment23';
+		$parentId = 'comment11.5';
+		$childrenCount = 6;
+		$message = 'I like to comment comment';
+		$verb = 'comment';
+		$actor = ['type' => 'user', 'id' => 'alice'];
+		$creationDT = new \DateTime();
+		$latestChildDT = new \DateTime('yesterday');
+		$object = ['type' => 'file', 'id' => 'file64'];
+
+		$comment
+			->setId($id)
+			->setParentId($parentId)
+			->setChildrenCount($childrenCount)
+			->setMessage($message)
+			->setVerb($verb)
+			->setActor($actor['type'], $actor['id'])
+			->setCreationDateTime($creationDT)
+			->setLatestChildDateTime($latestChildDT)
+			->setObject($object['type'], $object['id']);
+
+		$this->assertSame($id, $comment->getId());
+		$this->assertSame($parentId, $comment->getParentId());
+		$this->assertSame($childrenCount, $comment->getChildrenCount());
+		$this->assertSame($message, $comment->getMessage());
+		$this->assertSame($verb, $comment->getVerb());
+		$this->assertSame($actor['type'], $comment->getActorType());
+		$this->assertSame($actor['id'], $comment->getActorId());
+		$this->assertSame($creationDT, $comment->getCreationDateTime());
+		$this->assertSame($latestChildDT, $comment->getLatestChildDateTime());
+		$this->assertSame($object['type'], $comment->getObjectType());
+		$this->assertSame($object['id'], $comment->getObjectId());
+	}
+
+	public function testSetIdIllegalInput() {
+		$comment = new \OC\Comments\Comment();
+
+		$this->setExpectedException('\OCP\Comments\IllegalIDChangeException');
+		$comment->setId('c23');
+		$comment->setId('c17');
+	}
+
+	public function testResetId() {
+		$comment = new \OC\Comments\Comment();
+		$comment->setId('c23');
+		$comment->setId('');
+	}
+
+	public function simpleSetterProvider() {
+		return [
+			['Id'],
+			['ParentId'],
+			['Message'],
+			['Verb'],
+			['ChildrenCount'],
+		];
+	}
+
+	/**
+	 * @dataProvider simpleSetterProvider
+	 */
+	public function testSimpleSetterInvalidInput($field) {
+		$comment = new \OC\Comments\Comment();
+		$setter = 'set' . $field;
+
+		$this->setExpectedException('InvalidArgumentException');
+		// we have no field that is supposed to accept a Bool
+		$comment->$setter(true);
+	}
+
+	public function roleSetterProvider() {
+		return [
+			['Actor', true, true],
+			['Actor', 'user', true],
+			['Actor', true, 'alice'],
+			['Object', true, true],
+			['Object', 'file', true],
+			['Object', true, 'file64'],
+		];
+	}
+
+	/**
+	 * @dataProvider roleSetterProvider
+	 */
+	public function testSetRoleInvalidInput($role, $type, $id){
+		$comment = new \OC\Comments\Comment();
+		$setter = 'set' . $role;
+		$this->setExpectedException('InvalidArgumentException');
+		$comment->$setter($type, $id);
+	}
+
+
+
+}
diff --git a/tests/lib/comments/fakefactory.php b/tests/lib/comments/fakefactory.php
new file mode 100644
index 00000000000..202b02f6411
--- /dev/null
+++ b/tests/lib/comments/fakefactory.php
@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * Class Test_Comments_FakeFactory
+ *
+ * this class does not contain any tests. It's sole purpose is to return
+ * a mock of \OCP\Comments\ICommentsManager when getManager() is called.
+ * For mock creation and auto-loading it extends Test\TestCase. I am, uh, really
+ * sorry for this hack.
+ */
+class Test_Comments_FakeFactory extends Test\TestCase implements \OCP\Comments\ICommentsManagerFactory {
+
+	public function testNothing() {
+		// If there would not be at least one test, phpunit would scream failure
+		// So we have one and skip it.
+		$this->markTestSkipped();
+	}
+
+	public function getManager() {
+		return $this->getMock('\OCP\Comments\ICommentsManager');
+	}
+}
diff --git a/tests/lib/comments/manager.php b/tests/lib/comments/manager.php
new file mode 100644
index 00000000000..610dfe51a47
--- /dev/null
+++ b/tests/lib/comments/manager.php
@@ -0,0 +1,560 @@
+<?php
+
+use OCP\Comments\ICommentsManager;
+
+/**
+ * Class Test_Comments_Manager
+ *
+ * @group DB
+ */
+class Test_Comments_Manager extends Test\TestCase
+{
+
+	public function setUp() {
+		parent::setUp();
+
+		$sql = \oc::$server->getDatabaseConnection()->getDatabasePlatform()->getTruncateTableSQL('`*PREFIX*comments`');
+		\oc::$server->getDatabaseConnection()->prepare($sql)->execute();
+	}
+
+	protected function addDatabaseEntry($parentId, $topmostParentId, $creationDT = null, $latestChildDT = null) {
+		if(is_null($creationDT)) {
+			$creationDT = new \DateTime();
+		}
+		if(is_null($latestChildDT)) {
+			$latestChildDT = new \DateTime('yesterday');
+		}
+
+		$qb = \oc::$server->getDatabaseConnection()->getQueryBuilder();
+		$qb
+			->insert('comments')
+			->values([
+					'parent_id'					=> $qb->createNamedParameter($parentId),
+					'topmost_parent_id' 		=> $qb->createNamedParameter($topmostParentId),
+					'children_count' 			=> $qb->createNamedParameter(2),
+					'actor_type' 				=> $qb->createNamedParameter('user'),
+					'actor_id' 					=> $qb->createNamedParameter('alice'),
+					'message' 					=> $qb->createNamedParameter('nice one'),
+					'verb' 						=> $qb->createNamedParameter('comment'),
+					'creation_timestamp' 		=> $qb->createNamedParameter($creationDT, 'datetime'),
+					'latest_child_timestamp'	=> $qb->createNamedParameter($latestChildDT, 'datetime'),
+					'object_type' 				=> $qb->createNamedParameter('file'),
+					'object_id' 				=> $qb->createNamedParameter('file64'),
+			])
+			->execute();
+
+		return \oc::$server->getDatabaseConnection()->lastInsertId('*PREFIX*comments');
+	}
+
+	protected function getManager() {
+		$factory = new \OC\Comments\ManagerFactory();
+		return $factory->getManager();
+	}
+
+	public function testGetCommentNotFound() {
+		$manager = $this->getManager();
+		$this->setExpectedException('\OCP\Comments\NotFoundException');
+		$manager->get('22');
+	}
+
+	public function testGetCommentNotFoundInvalidInput() {
+		$manager = $this->getManager();
+		$this->setExpectedException('\InvalidArgumentException');
+		$manager->get('unexisting22');
+	}
+
+	public function testGetComment() {
+		$manager = $this->getManager();
+
+		$creationDT = new \DateTime();
+		$latestChildDT = new \DateTime('yesterday');
+
+		$qb = \oc::$server->getDatabaseConnection()->getQueryBuilder();
+		$qb
+			->insert('comments')
+			->values([
+					'parent_id'					=> $qb->createNamedParameter('2'),
+					'topmost_parent_id' 		=> $qb->createNamedParameter('1'),
+					'children_count' 			=> $qb->createNamedParameter(2),
+					'actor_type' 				=> $qb->createNamedParameter('user'),
+					'actor_id' 					=> $qb->createNamedParameter('alice'),
+					'message' 					=> $qb->createNamedParameter('nice one'),
+					'verb' 						=> $qb->createNamedParameter('comment'),
+					'creation_timestamp' 		=> $qb->createNamedParameter($creationDT, 'datetime'),
+					'latest_child_timestamp'	=> $qb->createNamedParameter($latestChildDT, 'datetime'),
+					'object_type' 				=> $qb->createNamedParameter('file'),
+					'object_id' 				=> $qb->createNamedParameter('file64'),
+			])
+			->execute();
+
+		$id = strval(\oc::$server->getDatabaseConnection()->lastInsertId('*PREFIX*comments'));
+
+		$comment = $manager->get($id);
+		$this->assertTrue($comment instanceof \OCP\Comments\IComment);
+		$this->assertSame($comment->getId(), $id);
+		$this->assertSame($comment->getParentId(), '2');
+		$this->assertSame($comment->getTopmostParentId(), '1');
+		$this->assertSame($comment->getChildrenCount(), 2);
+		$this->assertSame($comment->getActorType(), 'user');
+		$this->assertSame($comment->getActorId(), 'alice');
+		$this->assertSame($comment->getMessage(), 'nice one');
+		$this->assertSame($comment->getVerb(), 'comment');
+		$this->assertSame($comment->getObjectType(), 'file');
+		$this->assertSame($comment->getObjectId(), 'file64');
+		$this->assertEquals($comment->getCreationDateTime(), $creationDT);
+		$this->assertEquals($comment->getLatestChildDateTime(), $latestChildDT);
+	}
+
+	public function testGetTreeNotFound() {
+		$manager = $this->getManager();
+		$this->setExpectedException('\OCP\Comments\NotFoundException');
+		$manager->getTree('22');
+	}
+
+	public function testGetTreeNotFoundInvalidIpnut() {
+		$manager = $this->getManager();
+		$this->setExpectedException('\InvalidArgumentException');
+		$manager->getTree('unexisting22');
+	}
+
+	public function testGetTree() {
+		$headId = $this->addDatabaseEntry(0, 0);
+
+		$this->addDatabaseEntry($headId, $headId, new \DateTime('-3 hours'));
+		$this->addDatabaseEntry($headId, $headId, new \DateTime('-2 hours'));
+		$id = $this->addDatabaseEntry($headId, $headId, new \DateTime('-1 hour'));
+
+		$manager = $this->getManager();
+		$tree = $manager->getTree($headId);
+
+		// Verifying the root comment
+		$this->assertTrue(isset($tree['comment']));
+		$this->assertTrue($tree['comment'] instanceof \OCP\Comments\IComment);
+		$this->assertSame($tree['comment']->getId(), strval($headId));
+		$this->assertTrue(isset($tree['replies']));
+		$this->assertSame(count($tree['replies']), 3);
+
+		// one level deep
+		foreach($tree['replies'] as $reply) {
+			$this->assertTrue($reply['comment'] instanceof \OCP\Comments\IComment);
+			$this->assertSame($reply['comment']->getId(), strval($id));
+			$this->assertSame(count($reply['replies']), 0);
+			$id--;
+		}
+	}
+
+	public function testGetTreeNoReplies() {
+		$id = $this->addDatabaseEntry(0, 0);
+
+		$manager = $this->getManager();
+		$tree = $manager->getTree($id);
+
+		// Verifying the root comment
+		$this->assertTrue(isset($tree['comment']));
+		$this->assertTrue($tree['comment'] instanceof \OCP\Comments\IComment);
+		$this->assertSame($tree['comment']->getId(), strval($id));
+		$this->assertTrue(isset($tree['replies']));
+		$this->assertSame(count($tree['replies']), 0);
+
+		// one level deep
+		foreach($tree['replies'] as $reply) {
+			throw new \Exception('This ain`t happen');
+		}
+	}
+
+	public function testGetTreeWithLimitAndOffset() {
+		$headId = $this->addDatabaseEntry(0, 0);
+
+		$this->addDatabaseEntry($headId, $headId, new \DateTime('-3 hours'));
+		$this->addDatabaseEntry($headId, $headId, new \DateTime('-2 hours'));
+		$this->addDatabaseEntry($headId, $headId, new \DateTime('-1 hour'));
+		$idToVerify = $this->addDatabaseEntry($headId, $headId, new \DateTime());
+
+		$manager = $this->getManager();
+
+		for ($offset = 0; $offset < 3; $offset += 2) {
+			$tree = $manager->getTree(strval($headId), 2, $offset);
+
+			// Verifying the root comment
+			$this->assertTrue(isset($tree['comment']));
+			$this->assertTrue($tree['comment'] instanceof \OCP\Comments\IComment);
+			$this->assertSame($tree['comment']->getId(), strval($headId));
+			$this->assertTrue(isset($tree['replies']));
+			$this->assertSame(count($tree['replies']), 2);
+
+			// one level deep
+			foreach ($tree['replies'] as $reply) {
+				$this->assertTrue($reply['comment'] instanceof \OCP\Comments\IComment);
+				$this->assertSame($reply['comment']->getId(), strval($idToVerify));
+				$this->assertSame(count($reply['replies']), 0);
+				$idToVerify--;
+			}
+		}
+	}
+
+	public function testGetForObject() {
+		$this->addDatabaseEntry(0, 0);
+
+		$manager = $this->getManager();
+		$comments = $manager->getForObject('file', 'file64');
+
+		$this->assertTrue(is_array($comments));
+		$this->assertSame(count($comments), 1);
+		$this->assertTrue($comments[0] instanceof \OCP\Comments\IComment);
+		$this->assertSame($comments[0]->getMessage(), 'nice one');
+	}
+
+	public function testGetForObjectWithLimitAndOffset() {
+		$this->addDatabaseEntry(0, 0, new \DateTime('-6 hours'));
+		$this->addDatabaseEntry(0, 0, new \DateTime('-5 hours'));
+		$this->addDatabaseEntry(1, 1, new \DateTime('-4 hours'));
+		$this->addDatabaseEntry(0, 0, new \DateTime('-3 hours'));
+		$this->addDatabaseEntry(2, 2, new \DateTime('-2 hours'));
+		$this->addDatabaseEntry(2, 2, new \DateTime('-1 hours'));
+		$idToVerify = $this->addDatabaseEntry(3, 1, new \DateTime());
+
+		$manager = $this->getManager();
+		$offset = 0;
+		do {
+			$comments = $manager->getForObject('file', 'file64', 3, $offset);
+
+			$this->assertTrue(is_array($comments));
+			foreach($comments as $comment) {
+				$this->assertTrue($comment instanceof \OCP\Comments\IComment);
+				$this->assertSame($comment->getMessage(), 'nice one');
+				$this->assertSame($comment->getId(), strval($idToVerify));
+				$idToVerify--;
+			}
+			$offset += 3;
+		} while(count($comments) > 0);
+	}
+
+	public function testGetForObjectWithDateTimeConstraint() {
+		$this->addDatabaseEntry(0, 0, new \DateTime('-6 hours'));
+		$this->addDatabaseEntry(0, 0, new \DateTime('-5 hours'));
+		$id1 = $this->addDatabaseEntry(0, 0, new \DateTime('-3 hours'));
+		$id2 = $this->addDatabaseEntry(2, 2, new \DateTime('-2 hours'));
+
+		$manager = $this->getManager();
+		$comments = $manager->getForObject('file', 'file64', 0, 0, new \DateTime('-4 hours'));
+
+		$this->assertSame(count($comments), 2);
+		$this->assertSame($comments[0]->getId(), strval($id2));
+		$this->assertSame($comments[1]->getId(), strval($id1));
+	}
+
+	public function testGetForObjectWithLimitAndOffsetAndDateTimeConstraint() {
+		$this->addDatabaseEntry(0, 0, new \DateTime('-7 hours'));
+		$this->addDatabaseEntry(0, 0, new \DateTime('-6 hours'));
+		$this->addDatabaseEntry(1, 1, new \DateTime('-5 hours'));
+		$this->addDatabaseEntry(0, 0, new \DateTime('-3 hours'));
+		$this->addDatabaseEntry(2, 2, new \DateTime('-2 hours'));
+		$this->addDatabaseEntry(2, 2, new \DateTime('-1 hours'));
+		$idToVerify = $this->addDatabaseEntry(3, 1, new \DateTime());
+
+		$manager = $this->getManager();
+		$offset = 0;
+		do {
+			$comments = $manager->getForObject('file', 'file64', 3, $offset, new \DateTime('-4 hours'));
+
+			$this->assertTrue(is_array($comments));
+			foreach($comments as $comment) {
+				$this->assertTrue($comment instanceof \OCP\Comments\IComment);
+				$this->assertSame($comment->getMessage(), 'nice one');
+				$this->assertSame($comment->getId(), strval($idToVerify));
+				$this->assertTrue(intval($comment->getId()) >= 4);
+				$idToVerify--;
+			}
+			$offset += 3;
+		} while(count($comments) > 0);
+	}
+
+	public function testGetNumberOfCommentsForObject() {
+		for($i = 1; $i < 5; $i++) {
+			$this->addDatabaseEntry(0, 0);
+		}
+
+		$manager = $this->getManager();
+
+		$amount = $manager->getNumberOfCommentsForObject('untype', '00');
+		$this->assertSame($amount, 0);
+
+		$amount = $manager->getNumberOfCommentsForObject('file', 'file64');
+		$this->assertSame($amount, 4);
+	}
+
+	public function invalidCreateArgsProvider() {
+		return [
+			['', 'aId-1', 'oType-1', 'oId-1'],
+			['aType-1', '', 'oType-1', 'oId-1'],
+			['aType-1', 'aId-1', '', 'oId-1'],
+			['aType-1', 'aId-1', 'oType-1', ''],
+			[1, 'aId-1', 'oType-1', 'oId-1'],
+			['aType-1', 1, 'oType-1', 'oId-1'],
+			['aType-1', 'aId-1', 1, 'oId-1'],
+			['aType-1', 'aId-1', 'oType-1', 1],
+		];
+	}
+
+	/**
+	 * @dataProvider invalidCreateArgsProvider
+	 */
+	public function testCreateCommentInvalidArguments($aType, $aId, $oType, $oId) {
+		$manager = $this->getManager();
+		$this->setExpectedException('\InvalidArgumentException');
+		$manager->create($aType, $aId, $oType, $oId);
+	}
+
+	public function testCreateComment() {
+		$actorType = 'bot';
+		$actorId = 'bob';
+		$objectType = 'weather';
+		$objectId = 'bielefeld';
+
+		$comment = $this->getManager()->create($actorType, $actorId, $objectType, $objectId);
+		$this->assertTrue($comment instanceof \OCP\Comments\IComment);
+		$this->assertSame($comment->getActorType(), $actorType);
+		$this->assertSame($comment->getActorId(), $actorId);
+		$this->assertSame($comment->getObjectType(), $objectType);
+		$this->assertSame($comment->getObjectId(), $objectId);
+	}
+
+	public function testDelete() {
+		$manager = $this->getManager();
+
+		$done = $manager->delete('404');
+		$this->assertFalse($done);
+
+		$done = $manager->delete('%');
+		$this->assertFalse($done);
+
+		$done = $manager->delete('');
+		$this->assertFalse($done);
+
+		$id = strval($this->addDatabaseEntry(0, 0));
+		$comment = $manager->get($id);
+		$this->assertTrue($comment instanceof \OCP\Comments\IComment);
+		$done = $manager->delete($id);
+		$this->assertTrue($done);
+		$this->setExpectedException('\OCP\Comments\NotFoundException');
+		$manager->get($id);
+	}
+
+	public function testSaveNew() {
+		$manager = $this->getManager();
+		$comment = new \OC\Comments\Comment();
+		$comment
+			->setActor('user', 'alice')
+			->setObject('file', 'file64')
+			->setMessage('very beautiful, I am impressed!')
+			->setVerb('comment');
+
+		$saveSuccessful = $manager->save($comment);
+		$this->assertTrue($saveSuccessful);
+		$this->assertTrue($comment->getId() !== '');
+		$this->assertTrue($comment->getId() !== '0');
+		$this->assertTrue(!is_null($comment->getCreationDateTime()));
+
+		$loadedComment = $manager->get($comment->getId());
+		$this->assertSame($comment->getMessage(), $loadedComment->getMessage());
+		$this->assertEquals($comment->getCreationDateTime(), $loadedComment->getCreationDateTime());
+	}
+
+	public function testSaveUpdate() {
+		$manager = $this->getManager();
+		$comment = new \OC\Comments\Comment();
+		$comment
+				->setActor('user', 'alice')
+				->setObject('file', 'file64')
+				->setMessage('very beautiful, I am impressed!')
+				->setVerb('comment');
+
+		$manager->save($comment);
+
+		$comment->setMessage('very beautiful, I am really so much impressed!');
+		$manager->save($comment);
+
+		$loadedComment = $manager->get($comment->getId());
+		$this->assertSame($comment->getMessage(), $loadedComment->getMessage());
+	}
+
+	public function testSaveUpdateException() {
+		$manager = $this->getManager();
+		$comment = new \OC\Comments\Comment();
+		$comment
+				->setActor('user', 'alice')
+				->setObject('file', 'file64')
+				->setMessage('very beautiful, I am impressed!')
+				->setVerb('comment');
+
+		$manager->save($comment);
+
+		$manager->delete($comment->getId());
+		$comment->setMessage('very beautiful, I am really so much impressed!');
+		$this->setExpectedException('\OCP\Comments\NotFoundException');
+		$manager->save($comment);
+	}
+
+	public function testSaveIncomplete() {
+		$manager = $this->getManager();
+		$comment = new \OC\Comments\Comment();
+		$comment->setMessage('from no one to nothing');
+		$this->setExpectedException('\UnexpectedValueException');
+		$manager->save($comment);
+	}
+
+	public function testSaveAsChild() {
+		$id = $this->addDatabaseEntry(0, 0);
+
+		$manager = $this->getManager();
+
+		for($i = 0; $i < 3; $i++) {
+			$comment = new \OC\Comments\Comment();
+			$comment
+					->setActor('user', 'alice')
+					->setObject('file', 'file64')
+					->setParentId(strval($id))
+					->setMessage('full ack')
+					->setVerb('comment')
+					// setting the creation time avoids using sleep() while making sure to test with different timestamps
+					->setCreationDateTime(new \DateTime('+' . $i . ' minutes'));
+
+			$manager->save($comment);
+
+			$this->assertSame($comment->getTopmostParentId(), strval($id));
+			$parentComment = $manager->get(strval($id));
+			$this->assertSame($parentComment->getChildrenCount(), $i + 1);
+			$this->assertEquals($parentComment->getLatestChildDateTime(), $comment->getCreationDateTime());
+		}
+	}
+
+	public function invalidActorArgsProvider() {
+		return
+		[
+			['', ''],
+			[1, 'alice'],
+			['user', 1],
+		];
+	}
+
+	/**
+	 * @dataProvider invalidActorArgsProvider
+	 */
+	public function testDeleteReferencesOfActorInvalidInput($type, $id) {
+		$manager = $this->getManager();
+		$this->setExpectedException('\InvalidArgumentException');
+		$manager->deleteReferencesOfActor($type, $id);
+	}
+
+	public function testDeleteReferencesOfActor() {
+		$ids = [];
+		$ids[] = $this->addDatabaseEntry(0, 0);
+		$ids[] = $this->addDatabaseEntry(0, 0);
+		$ids[] = $this->addDatabaseEntry(0, 0);
+
+		$manager = $this->getManager();
+
+		// just to make sure they are really set, with correct actor data
+		$comment = $manager->get(strval($ids[1]));
+		$this->assertSame($comment->getActorType(), 'user');
+		$this->assertSame($comment->getActorId(), 'alice');
+
+		$wasSuccessful = $manager->deleteReferencesOfActor('user', 'alice');
+		$this->assertTrue($wasSuccessful);
+
+		foreach($ids as $id) {
+			$comment = $manager->get(strval($id));
+			$this->assertSame($comment->getActorType(), ICommentsManager::DELETED_USER);
+			$this->assertSame($comment->getActorId(), ICommentsManager::DELETED_USER);
+		}
+
+		// actor info is gone from DB, but when database interaction is alright,
+		// we still expect to get true back
+		$wasSuccessful = $manager->deleteReferencesOfActor('user', 'alice');
+		$this->assertTrue($wasSuccessful);
+	}
+
+	public function testDeleteReferencesOfActorWithUserManagement() {
+		$user = \oc::$server->getUserManager()->createUser('xenia', '123456');
+		$this->assertTrue($user instanceof \OCP\IUser);
+
+		$manager = $this->getManager();
+		$comment = $manager->create('user', $user->getUID(), 'file', 'file64');
+		$comment
+			->setMessage('Most important comment I ever left on the Internet.')
+			->setVerb('comment');
+		$status = $manager->save($comment);
+		$this->assertTrue($status);
+
+		$commentID = $comment->getId();
+		$user->delete();
+
+		$comment =$manager->get($commentID);
+		$this->assertSame($comment->getActorType(), \OCP\Comments\ICommentsManager::DELETED_USER);
+		$this->assertSame($comment->getActorId(), \OCP\Comments\ICommentsManager::DELETED_USER);
+	}
+
+	public function invalidObjectArgsProvider() {
+		return
+				[
+						['', ''],
+						[1, 'file64'],
+						['file', 1],
+				];
+	}
+
+	/**
+	 * @dataProvider invalidObjectArgsProvider
+	 */
+	public function testDeleteCommentsAtObjectInvalidInput($type, $id) {
+		$manager = $this->getManager();
+		$this->setExpectedException('\InvalidArgumentException');
+		$manager->deleteCommentsAtObject($type, $id);
+	}
+
+	public function testDeleteCommentsAtObject() {
+		$ids = [];
+		$ids[] = $this->addDatabaseEntry(0, 0);
+		$ids[] = $this->addDatabaseEntry(0, 0);
+		$ids[] = $this->addDatabaseEntry(0, 0);
+
+		$manager = $this->getManager();
+
+		// just to make sure they are really set, with correct actor data
+		$comment = $manager->get(strval($ids[1]));
+		$this->assertSame($comment->getObjectType(), 'file');
+		$this->assertSame($comment->getObjectId(), 'file64');
+
+		$wasSuccessful = $manager->deleteCommentsAtObject('file', 'file64');
+		$this->assertTrue($wasSuccessful);
+
+		$verified = 0;
+		foreach($ids as $id) {
+			try {
+				$manager->get(strval($id));
+			} catch (\OCP\Comments\NotFoundException $e) {
+				$verified++;
+			}
+		}
+		$this->assertSame($verified, 3);
+
+		// actor info is gone from DB, but when database interaction is alright,
+		// we still expect to get true back
+		$wasSuccessful = $manager->deleteCommentsAtObject('file', 'file64');
+		$this->assertTrue($wasSuccessful);
+	}
+
+	public function testOverwriteDefaultManager() {
+		$config = \oc::$server->getConfig();
+		$defaultManagerFactory = $config->getSystemValue('comments.managerFactory', '\OC\Comments\ManagerFactory');
+
+		$managerMock = $this->getMock('\OCP\Comments\ICommentsManager');
+
+		$config->setSystemValue('comments.managerFactory', 'Test_Comments_FakeFactory');
+		$manager = \oc::$server->getCommentsManager();
+		$this->assertEquals($managerMock, $manager);
+
+		$config->setSystemValue('comments.managerFactory', $defaultManagerFactory);
+	}
+
+}
diff --git a/tests/lib/server.php b/tests/lib/server.php
index b72bef82036..0bee19822d2 100644
--- a/tests/lib/server.php
+++ b/tests/lib/server.php
@@ -61,6 +61,7 @@ class Server extends \Test\TestCase {
 			['CapabilitiesManager', '\OC\CapabilitiesManager'],
 			['ContactsManager', '\OC\ContactsManager'],
 			['ContactsManager', '\OCP\Contacts\IManager'],
+			['CommentsManager', '\OCP\Comments\ICommentsManager'],
 			['Crypto', '\OC\Security\Crypto'],
 			['Crypto', '\OCP\Security\ICrypto'],
 			['CryptoWrapper', '\OC\Session\CryptoWrapper'],