title: Delete an entity without to delete the foreign key link to it
body: <p>I have a table <em>Products</em> the entity <em>tva</em> is link to the table <em>Tva</em> and the entity <em>image</em> is link to the table <em>Media</em>. I render in sonata admin all the products of the website. I would like to be able to delete a product from the administration. </p>
  
  <p>I want be able to delete a product without to delete the foreign key link to it.</p>
  
  <p>But i have this error: </p>
  
  <blockquote>
    <p>SQLSTATE[23000]: Integrity constraint violation: 1451 Cannot delete or
    update a parent row: a foreign key constraint fails (<code>fly</code>.<code>post</code>,
    CONSTRAINT <code>FK_5A8A6C8D4D79775F</code> FOREIGN KEY (<code>tva_id</code>) REFERENCES
    <code>tva</code> (<code>id</code>))</p>
  </blockquote>
  
  <p>This is my entity tva and image</p>
  
  <pre><code>/**
   * @ORM\ManyToOne(targetEntity="FLY\BookingsBundle\Entity\Tva", cascade={"persist", "remove"})
   * @ORM\JoinColumn(nullable=true)
   */
  
  private $tva;
  
  /**
   * @ORM\ManyToOne(targetEntity="FLY\BookingsBundle\Entity\Media", cascade={"persist","remove"})
   * @ORM\JoinColumn(nullable=true)
   */
  private $image;
  </code></pre>
  
  <p>I decided to change the join column from this    <code>* @ORM\JoinColumn(nullable=true)</code>  to this <code>* @ORM\JoinColumn(onDelete="CASCADE")</code>  and it works i can delete the product but it also delete the foreign key and i don't want that because i have other product that use the same image and tva.</p>