added DiffTask Entity and Mapper

lib/Db/DiffTask.php

@@ -0,0 +1,28 @@
+<?php
+namespace OCA\GroupfolderFilesystemSnapshots\Db;
+
+use JsonSerializable;
+
+use OCP\AppFramework\Db\Entity;
+
+class DiffTask extends Entity implements JsonSerializable {
+
+    protected $userId;
+    protected $groupfolderId;
+    protected $snapshotId;
+    protected $timestamp;
+
+    public function __construct() {
+        $this->addType('id','integer');
+        $this->addType('groupfolderId','integer');
+    }
+
+    public function jsonSerialize() {
+        return [
+            'id' => $this->id,
+            'groupfolderId' => $this->groupfolderId,
+            'snapshotId' => $this->snapshotId,
+            'timestamp' => $this->timestamp,
+        ];
+    }
+}

lib/Db/DiffTaskMapper.php

@@ -0,0 +1,37 @@
+<?php
+namespace OCA\GroupfolderFilesystemSnapshots\Db;
+
+use OCP\IDBConnection;
+use OCP\AppFramework\Db\QBMapper;
+
+/**
+ * @extends QBMapper<DiffTask>
+ */
+class DiffTaskMapper extends QBMapper {
+
+    public function __construct(IDBConnection $db) {
+        parent::__construct($db, 'groupfolder_snapshots_tasks', DiffTask::class);
+    }
+
+    public function find(int $id, string $userId) {
+        $qb = $this->db->getQueryBuilder();
+
+        $qb->select('*')
+             ->from($this->getTableName())
+             ->where($qb->expr()->eq('id', $qb->createNamedParameter($id)))
+             ->andWhere($qb->expr()->eq('user_id', $qb->createNamedParameter($userId)));
+
+        return $this->findEntity($qb);
+    }
+
+    public function findAll(string $userId) {
+        $qb = $this->db->getQueryBuilder();
+
+        $qb->select('*')
+           ->from($this->getTableName())
+           ->where($qb->expr()->eq('user_id', $qb->createNamedParameter($userId)));
+
+        return $this->findEntities($qb);
+    }
+
+}