title: ORM UniqueConstraint, null value duplicate
body: <p>I have an entity with an unique constraint on 3 fiels.</p>
  
  <p>2 fields are not nullable, and the third is nullable.</p>
  
  <p>My problem, when the third field is null, the unique constraint doesn't work, so i can have in my database some duplicate values like (<code>5,1,null)(5,1,null)</code></p>
  
  <p>For me, (<code>5,1,null)</code> is an unique value, with null too.</p>
  
  <p>Do you have any ideas ?</p>
  
  <p>This is my ORM line : </p>
  
  <pre><code>* @ORM\Table(name="table", uniqueConstraints={@ORM\UniqueConstraint(name="table_idx", columns={"field1", "field2", "field3"})})
  </code></pre>
  
  <p>Edit : The nullable value is a foreign key to an other entity, so i can't put a custom value. It's an entity or null.</p>