title: How to prevent Doctrine2 from cascade persisting an entity on event
body: <p>Let's say I have a simple entity EstablishmentEntity, with a ManyToOne relationship on $employee that looks like this :</p>
  
  <pre><code>namespace Msm\CeopBundle\Entity;
  use Doctrine\ORM\Mapping as ORM;
  use Msm\CeopBundle\Entity\Establishment;
  
  /**
   * Establishment
   *
   * @ORM\Table(name="establishment")
   * @ORM\Entity()
   */
  class School
  {
  
  /**
   * @var integer
   *
   * @ORM\Column(name="id", type="integer")
   * @ORM\Id
   * @ORM\GeneratedValue(strategy="AUTO")
   */
  private $id;
  
  /**
   *
   * @ORM\ManyToOne(targetEntity="Professionnal", cascade={"persist"})
   * @ORM\JoinColumn(name="teacher_id", referencedColumnName="id", nullable=true) 
   */
  private $employee;
  </code></pre>
  
  <p>My problem : that entity is used for different kinds of establishments and has a lot more useful attributes in my code (I kept it simple for my question). One kind of establishement won't have any employees. Is it possible to tell Doctrine/symfony NOT to cascade persist the employee in that perticular case after class instanciation for example ?</p>
  
  <p>So far it automatically persists an empty employee entity when creating an establishment, which is ok for most establishments but not all...</p>