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.

Updates to managed documents under Doctrine MongoDB ODM

If you update a document returned by Doctrine MongoDB ODM, changes to that document will be flushed to even if you do not call the persist() method on that document. That is because the document returned is a managed document. In fact, the persist() call gets ignored for these managed documents. From the documentation:

If X is a preexisting managed document, it is ignored by the persist operation. However, the persist operation is cascaded to documents referenced by X, if the relationships from X to these other documents are mapped with cascade=PERSIST or cascade=ALL.

In my case, this recently caused an issue where duplicate embedded documents ended up being pushed into a parent document because I was using and updating the parent document for generating a custom Symfony2 form. Turned out to be a bad idea... If you need to update these type of documents, detach them first.