原则2-禁止ManyToOne关系的外键上的空值

class License {
    // ...
    /**
     * Customer who owns the license
     * 
     * @var \\ISE\\LicenseManagerBundle\\Entity\\Customer
     * @ORM\\ManyToOne(targetEntity=\"Customer\", inversedBy=\"licenses\")
     * @ORM\\JoinColumn(name=\"customer_id\", referencedColumnName=\"id\")
     */
    private $customer;
    // ...
}

class Customer {
    // ...
    /**
     * Licenses that were at one point generated for the customer
     * 
     * @var \\Doctrine\\Common\\Collections\\ArrayCollection
     * @ORM\\OneToMany(targetEntity=\"License\", mappedBy=\"customer\")
     */
    private $licenses;
    // ...
}

$em = $this->get(\'doctrine\')->getEntityManager();
$license = new License();
// Set some fields - not the reference fields though
$license->setValidUntil(new \\DateTime(\"2012-12-31\"));
$license->setCreatedAt(new \\DateTime());
// Persist the object
$em->persist($license);
$em->flush();