User can submit new journal through API.

app/Factory/TransactionGroupFactory.php

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+<?php
+/**
+ * TransactionGroupFactory.php
+ * Copyright (c) 2019 thegrumpydictator@gmail.com
+ *
+ * This file is part of Firefly III.
+ *
+ * Firefly III is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Firefly III is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Firefly III. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+namespace FireflyIII\Factory;
+
+use Exception;
+use FireflyIII\Exceptions\FireflyException;
+use FireflyIII\Models\TransactionGroup;
+use FireflyIII\Models\TransactionJournal;
+use FireflyIII\User;
+
+/**
+ * Class TransactionGroupFactory
+ */
+class TransactionGroupFactory
+{
+    /** @var TransactionJournalFactory */
+    private $journalFactory;
+    /** @var User The user */
+    private $user;
+
+    /**
+     * TransactionGroupFactory constructor.
+     */
+    public function __construct()
+    {
+        $this->journalFactory = app(TransactionJournalFactory::class);
+    }
+
+    /**
+     * Store a new transaction journal.
+     *
+     * @param array $data
+     *
+     * @return TransactionGroup
+     * @throws FireflyException
+     */
+    public function create(array $data): TransactionGroup
+    {
+        $this->journalFactory->setUser($this->user);
+        $collection = $this->journalFactory->create($data);
+        $title      = $data['group_title'] ?? null;
+        /** @var TransactionJournal $first */
+        $first = $collection->first();
+        $group = new TransactionGroup;
+        $group->user()->associate($first->user);
+        $group->title = $title ?? $first->description;
+        $group->save();
+
+        $group->transactionJournals()->saveMany($collection);
+
+        return $group;
+    }
+
+    /**
+     * Set the user.
+     *
+     * @param User $user
+     */
+    public function setUser(User $user): void
+    {
+        $this->user = $user;
+    }
+}