Fields getting removed when updatating mongodb document - mongodb

Question

I'm new to mongoDB and I have stumbled over some update issues when using Save().
When I click the save-button in my html-page I run the following code:
<?php
$action = (!empty($_POST['btn_submit']) &&
($_POST['btn_submit'] === 'Save')) ? 'save_article' : 'show_form';
$id = $_REQUEST['id'];
try {
$mongodb = new Mongo();
$articleCollection = $mongodb->myblogsite->articles;
} catch (MongoConnectionException $e) {
die('Failed to connect to MongoDB ' . $e->getMessage());
}
switch ($action) {
case 'save_article':
$article = array();
if (!empty($id)) {
$article['_id'] = new MongoId($id);
} else {
$article['_id'] = new MongoId();
}
$article['title'] = $_POST['title'];
$article['content'] = $_POST['content'];
$article['tags'] = $_POST['tags'];
if (!empty($id)) {
$article['updated_at'] = new MongoDate();
} else {
$article['saved_at'] = new MongoDate();
}
$articleCollection->save($article);
break;
case 'show_form':
default:
if (!empty($id)) {
$article = $articleCollection->findOne(array('_id' => new MongoId($id)));
}
}
?>
The idea is that if $id is not empty I update the document or else I insert.
I use save() to mange that.
I want two date-fields. One that hold the date the document was made and a new one for when it was last updated.
The “save_at” can not be updated. The “updated_at” change every time an update is made.
My problem now is that when I try to add the “updated_at” the “save_at” gets removed.
I also update the “saved_at”-field.
My questions is:
How do I insert an new field ('updated_at') without removing the “saved_at”-field?
How do I keep the date in “saved_at”-field as it is without updating its value?

Answer 1

use can done it with update command
$collection->update(array('id' => id),array('$set' => array('updated_at' =>$article['updated_at'])));