在Moodle中写入数据库时​​出错

Question

我正在尝试为 moodle 中的学生提问自定义plugin。为此，我有两张桌子。 mdl_qadt and mdl_qads.我做了老师部分，也就是老师可以添加问题。这将保存到表mdl_qadt。现在学生登录并选择此活动显示表格。学生可以在这里发布答案。发布给出错误的答案时：

写入数据库时​​出错。

但答案和相关数据已插入表格mdl_qads。那么为什么错误显示??

错误页面图片

学生表格的图片

View.php

echo $OUTPUT->heading($qadt->name);

if ($qadt->intro) { // Conditions to show the intro can change to look for own settings or whatever
    echo $OUTPUT->box(format_module_intro('qadt', $qadt, $cm->id), 'generalbox mod_introbox', 'qadtintro');
}
echo "<b>". $qadt->question ."</b>";

require_once(dirname(__FILE__).'/stud_form.php');

    $stud_form = new student_form("stud_action.php?id={$cm->id}&ref={$qadt->id}");

    $stud_form->display();

echo $OUTPUT->footer();

stud_form.php

require_once($CFG->dirroot.'/lib/formslib.php');

    class student_form extends moodleform {

        function definition () {
            $mform =& $this->_form;
            //$mform->addElement('header', 'general', 'general');
            $mform->addElement('text', 'answer', 'Your answer');
            $mform->setType('answer', PARAM_TEXT);
            $mform->addRule('answer', null, 'required', null, 'client');
            $this->add_action_buttons(false, 'submit');
        }

    }

stud_action.php

require_once(dirname(dirname(dirname(__FILE__))).'/config.php');
require_once(dirname(__FILE__).'/lib.php');
require_once($CFG->dirroot.'/lib/formslib.php');

$answer = $_REQUEST['answer'];  //answer given by student or user
if(isset($_GET['ref']))
{
    $refid = $_GET['ref'];  // refid
}
if(isset($_GET['id']))
{
    $modid = $_GET['id'];  // module id
}
$currentTime = date('Y-m-d H:i:s');
$record = new stdClass();
$record->sid     = $USER->id;   // logged in user's id
$record->refid   = $refid;
$record->answer  = $answer;
$record->modified_date  = $currentTime;

$table = "qads";
$data = studResponse($record, $table);
function studResponse($record, $table)
{
    global $DB;
    $lastinsertid = $DB->insert_record($table, $record);
    return $lastinsertid;
}
if($data)
{
    header("location:view.php?id=$modid");
}

即使将数据插入表中，我也会收到错误。我怎样才能解决这个问题。？任何帮助都很明显。

修改

我收到以下错误

Debug info: unknown error fetching inserted id

[NULL] 
Error code: dmlwriteexception

Stack trace: •line 1101 of \lib\dml\mysqli_native_moodle_database.php: dml_write_exception thrown
•line 1144 of \lib\dml\mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->insert_record_raw()
•line 52 of \mod\qadt\locallib.php: call to mysqli_native_moodle_database->insert_record()
•line 18 of \mod\qadt\stud_action.php: call to students_response()