Doctrine: Value of mappedBy ignored in OneToMany A

I set up a simple User->Comment association in Doctrine, simple OneToMany (One User can write many Comments).

All works find, but i found a strange behavior of Doctrine. First some Code:

User Entity

/**
 * @ORM\Entity
 * @ORM\Table(name="users")
 */
class User extends EntityAbstract {
    /**
     * @var int
     * @ORM\Id
     * @ORM\Column(name="id", type="integer")
     * @ORM\GeneratedValue
     */
    protected $_id;

    /**
     * @var \Doctrine\Common\Collections\Collection
     * @ORM\OneToMany(targetEntity="Comment", mappedBy="TodayIsASunnyDay")
     *                                                  ^^^^^^^^^^^^^^^^ WTF
     */
    protected $_commentsAuthored;

    public function __construct() {
        $this->_commentsAuthored = 
            new \Doctrine\Common\Collections\ArrayCollection();
    }
}

Comment Entity

/**
 * @ORM\Entity
 * @ORM\Table(name="comments")
 */
class Comment extends EntityAbstract {
    /**
     * @var int
     * @ORM\Id
     * @ORM\Column(name="id", type="integer")
     * @ORM\GeneratedValue
     */
    protected $_id;

    /**
     * @var User
     * @ORM\ManyToOne(targetEntity="User")
     * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
     *
     */
    protected $_author;
}

In User, doctrine wants a mappedBy Attribute since its the inversed Side of the association. But as it seems, I can write anything I want as value. (the right value would be "user_id" i think?)

So, when is this value ever used or checked?

Yeah you're right, checked it in my project - no error is issued. It affects the way the entities are loaded (set fetch="EAGER" inside OneToMany and you'll get your error), mainly it sets the owner side on a comment object when hydrating data (there's a bit more magic going under the hood which is hard to understand). I'd post an issue in doctrine bug tracker, it's a source of errors that are hard to debug if you misspelled a field name.

You need to specify inversedBy as well:

/**
 * @var User
 * @ORM\ManyToOne(targetEntity="User", inversedBy="_commentsAuthored")
 * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
 *
 */
protected $_author;

~~It should be used when you do retrieve the value of _commentsAuthored and try to do something with it (e.g., count it)~~