Doctrine: authorize NULL in a foreign composite key - symfony

Question

I have the following entity:
/**
* SeriesAuthorRole
*
* #ORM\Table()
* #ORM\Entity(repositoryClass="Blog\Bundle\CoreBundle\Entity\SeriesAuthorRoleRepository")
*/
class SeriesAuthorRole extends AuthorRoleAbstract
{
/**
* #var Series
*
* #ORM\ManyToOne(targetEntity="Blog\Bundle\CoreBundle\Entity\Series", inversedBy="authors")
* #ORM\JoinColumn(name="series", referencedColumnName="id", nullable=false)
* #ORM\Id
*/
private $series;
/**
* #var Author
*
* #ORM\ManyToOne(targetEntity="Blog\Bundle\CoreBundle\Entity\Author")
* #ORM\JoinColumn(name="author", referencedColumnName="id", nullable=false)
* #ORM\Id
*/
protected $author;
/**
* #var Role
*
* #todo Must be nullable
*
* #ORM\ManyToOne(targetEntity="Blog\Bundle\CoreBundle\Entity\Role")
* #ORM\JoinColumn(name="role", referencedColumnName="id", nullable=true)
* #ORM\Id
*/
protected $role;
// ... Getters, setters
}
The idea behind it is quite simple: We have author, role and series entities. A series can have several authors with various roles. A same author can fulfill multiple roles in a series.
Sometimes, we don't know exactly what was the role of the author. In this case, the NULL value will be used for the role, the NULL value standing for "I don't know".
I was taught not to use NULL in foreign composite keys unless it has meaning. Well, it has meaning here, and I know that this could be implemented without Doctrine. However, for now, Symfony 2 throws that error:
Entity of type Blog\Bundle\CoreBundle\Entity\BandAuthorRole is missing an assigned ID for field 'role'. The identifier generation strategy for this entity requires the ID field to be populated before EntityManager#persist() is called. If you want automatically generated identifiers instead you need to adjust the metadata mapping accordingly.
500 Internal Server Error - ORMException
So how can I authorize NULL values in foreign composite keys ? Is it possible at all with Doctrine ?

Answer 1

Your #JoinColumn annotation is correct with referencing to http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/annotations-reference.html#annref-joincolumn
However,
Every entity with a composite key cannot use an id generator other
than “ASSIGNED”. That means the ID fields have to have their values
set before you call EntityManager#persist($entity).
http://docs.doctrine-project.org/en/2.0.x/tutorials/composite-primary-keys.html#general-considerations