Doctrine：为什么$ entity-> ManyToOneRelation-> getId（）会向数据库发出查询？

I have a User class with referrer field:

class User {
    /**
     * @ORM\ManyToOne(targetEntity="User", cascade={"detach"})
     * @ORM\JoinColumn(name="referrer", referencedColumnName="id")
     */
    protected $referrer = null;

    public function getReferer(){
        return $this->referrer;
    }
}

What have I read is that since Doctrine 2.2 when I call $user->getReferer()->getId() I should get referenced id without doctrine firing additional query.

But in my case, doctrine is fetching whole entity from database. What am I doing wrong?

I use symfony 2.7.1 and I run echo \Doctrine\ORM\Version::VERSION to check doctrine's version.

($user->getReferrer() isn't firing any additional query yet.)

EDIT:

How I test it:

$user = $this->getUser();
$referrer = $user->getReferrer();
//Until there one query is executed by security(?) to fetch authenticated user.
//Referrer is not yet initialized.

$this->addFlash('info', 'Referrer id: '.$referrer->getId());
//Now second query was fired. And $refferer is now initialized.

return array('data' => $referrer); //return $refferer object for twig dump.

Twig dumps something like this for code without getId():

User {#1184 ▼
    +__isInitialized__: false
    #id: "2"
    #referrer: null
}

And this for code with getId() used on $referrer:

User {#1184 ▼
    +__isInitialized__: true
    #id: 2      
    #referrer: null
}