Get id of the MongoDB document created in a Symfony3 repository

Question

In my Symfony3 project, I do have some documents such as User, described below:

UserBundle\Document\User:
    repositoryClass: UserBundle\Repository\UserRepository
    fields:
        userId:
            id: true
        email:
            type: string
        firstName:
            type: string
        lastName:
            type: string

Some time, I want to create a user document through the user repository. Working around the createQueryBuilder(), I have built this method:

public function insert($email, $firstName, $lastName)
{
    $data = array('email' => $email,
        'firstName' => $firstName,
        'lastName' => $lastName,
    );

    /* Create query */

    $query = $this->createQueryBuilder();

    /* Add Data */

    $query
        ->insert()
        ->setNewObj($data);

    /* Return */

    $query
        ->getQuery()
        ->execute();

    return true;
}

Rather than returning true, I would like to return the Id of the document I have just created.

Looking for answers, I found this piece of code:

$this->getDocumentManager()->getConnection()->lastInsertId

Unfortunately, it is not returning what I am looking for:

object(MongoDB)[307]
  public 'w' => int 1
  public 'wtimeout' => int 10000

Thanks for your help !

EDIT:

Here is an updated code with the suggestion made in the accepted answer:

public function insert($email, $firstName, $lastName)
{
    $id = new MongoId();

    $data = array(
        '_id' => $id,
        'email' => $email,
        'firstName' => $firstName,
        'lastName' => $lastName,
    );

    /* Create query */

    $query = $this->createQueryBuilder();

    /* Add Data */

    $query
        ->insert()
        ->setNewObj($data);

    /* Return */

    $query
        ->getQuery()
        ->execute();

    return $id->__toString();
}

Answer 1

The easiest way to solve your problem is to generate the identifier upfront by calling new \MongoId(), adding it to data you're inserting (under _id key) and return known value. Also this is exactly what underlying MongoDB driver does behind the scenes when you're inserting a document without identifier set explicitly (see this doc example)