Atomicly update an embedded document with Doctrine MongoDB ODM

I had to deal with an unexpected problem recently. In my setup, I have a Conversation document which contains multiple Message documents embedded inside - a one-to-many mapping basically - and I need to atomically push a new Message when a user replies to a conversation. Below is the initial code that I implemented:

public function reply($conversationId, Message $message, $flush = true)
{            
    $this->dm->createQueryBuilder($this->class)
        ->update()
        ->field('archivers')->unsetField()
        ->field('repliedBy')->set($message->getUserId())
        ->field('repliedBody')->set($message->getBody())
        ->field('repliedAt')->set(new \DateTime())
        ->field('modifiedAt')->set(new \DateTime())
        ->field('messages')->push($message)
        ->field('id')->equals(new \MongoId($conversationId))
        ->getQuery()
        ->execute();
}

Unfortunately, this approach failed miserably. Because if you use the push() call with a document instance, you get a Catchable Fatal Error: Object of class ... could not be converted to string in /vendor/bundles/Symfony/Bundle/DoctrineMongoDBBundle/Logger/DoctrineMongoDBLogger.php line 280 error.

After losing some hair over this, I decided to seek help. Luckily, jmikola (one of the developers of Doctrine MongoDB ODM) replied to my question at stackoverflow.com.

Turns out, the query builder is not really intended to work with document instances. As an alternative approach, I used an array representation of the Message document as illustrated below:

public function reply($conversationId, Message $message)
{            
    $this->dm->createQueryBuilder($this->class)
        ->update()
        ->field('archivers')->unsetField()
        ->field('repliedBy')->set($message->getUserId())
        ->field('repliedBody')->set($message->getBody())
        ->field('repliedAt')->set(new \DateTime())
        ->field('modifiedAt')->set(new \DateTime())
        ->field('messages')->push(array(
            '_id' => new \MongoId(),
            'userId' => $message->getuserId(),
            'body' => $message->getBody(),
            'createdAt' => new \DateTime(),
            'modifiedAt' => new \DateTime(),
        ))
        ->field('id')->equals(new \MongoId($conversationId))
        ->getQuery() 
        ->execute();
}

According to jmikola, you can also use the document manager to achieve the same result (see his last comment under his answer), but I have not verified this yet.