MDL-85018 qtype_multianswer: Prevent errors on backup/restore

If a multianswer question has its question_multianswer record missing
for some reason, attempting to backup and restore a bank containing the
question will result a dml_missing_record exception.

The qtype_multianswer::get_question_options() method already copes with
the sequence from this record being empty, so we can just remove the
MUST_EXIST check and let it fall back to that.

question/type/multianswer/questiontype.php

@@ -110,8 +110,7 @@ class qtype_multianswer extends question_type {
 
         parent::get_question_options($question);
         // Get relevant data indexed by positionkey from the multianswers table.
-        $sequence = $DB->get_field('question_multianswer', 'sequence',
-                array('question' => $question->id), MUST_EXIST);
+        $sequence = $DB->get_field('question_multianswer', 'sequence', ['question' => $question->id]);
 
         if (empty($sequence)) {
             $question->options->questions = [];

question/type/multianswer/tests/restore_test.php

@@ -0,0 +1,97 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle 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.
+//
+// Moodle 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 Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace qtype_multianswer;
+
+/**
+ * Unit tests for
+ *
+ * @package   qtype_multianswer
+ * @copyright 2025 onwards Catalyst IT EU {@link https://catalyst-eu.net}
+ * @author    Mark Johnson <[email protected]>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers \restore_qtype_multianswer_plugin
+ */
+final class restore_test extends \advanced_testcase {
+    /**
+     * Duplicate a quiz containing a multianswer question with no multianswer record.
+     */
+    public function test_restore_quiz_with_edited_questions(): void {
+        global $CFG, $DB, $USER;
+        require_once($CFG->dirroot . '/backup/util/includes/backup_includes.php');
+        require_once($CFG->dirroot . '/backup/util/includes/restore_includes.php');
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        // Create a course and a user with editing teacher capabilities.
+        $generator = $this->getDataGenerator();
+        $course1 = $generator->create_course();
+        $context = \context_course::instance($course1->id);
+        $questiongenerator = $this->getDataGenerator()->get_plugin_generator('core_question');
+        $initialcount = $DB->count_records('question');
+
+        // Create a question category.
+        $cat = $questiongenerator->create_question_category(['contextid' => $context->id]);
+
+        // Create a quiz containing a multianswer question from the qbank.
+        $quiz = $this->getDataGenerator()->get_plugin_generator('mod_quiz')->create_instance(['course' => $course1->id]);
+        $question = $questiongenerator->create_question('multianswer', 'twosubq', ['category' => $cat->id]);
+        quiz_add_quiz_question($question->id, $quiz);
+
+        // Delete the multianswer record.
+        $DB->delete_records('question_multianswer', ['question' => $question->id]);
+
+        // Confirm we have created 3 additional questions (one parent, 2 children).
+        $this->assertEquals($initialcount + 3, $DB->count_records('question'));
+
+        // Backup quiz.
+        $bc = new \backup_controller(
+            \backup::TYPE_1ACTIVITY,
+            $quiz->cmid,
+            \backup::FORMAT_MOODLE,
+            \backup::INTERACTIVE_NO,
+            \backup::MODE_IMPORT,
+            $USER->id,
+        );
+        $backupid = $bc->get_backupid();
+        $bc->execute_plan();
+        $bc->destroy();
+
+        // Restore the backup into the same course.
+        $rc = new \restore_controller(
+            $backupid,
+            $course1->id,
+            \backup::INTERACTIVE_NO,
+            \backup::MODE_IMPORT,
+            $USER->id,
+            \backup::TARGET_CURRENT_ADDING,
+        );
+        $rc->execute_precheck();
+        $rc->execute_plan();
+        $rc->destroy();
+
+        // Both quizzes should refer to the same original question.
+        $quizzes = get_fast_modinfo($course1->id)->get_instances_of('quiz');
+        $this->assertCount(2, $quizzes);
+        foreach ($quizzes as $quiz) {
+            $structure = \mod_quiz\question\bank\qbank_helper::get_question_structure($quiz->instance, $quiz->context);
+            $this->assertEquals($structure[1]->questionid, $question->id);
+        }
+
+        // There should be no additional questions created during the restore.
+        $this->assertEquals($initialcount + 3, $DB->count_records('question'));
+    }
+}