title: Doctrine MappedSuperclass and unique constraints
body: <p>I have a scenario where I need to use the MappedSuperclass functionality of Doctrine (using Symfony2), and also create a unique constraint on some superclass columns. Let's say:</p>
  
  <pre><code>use Doctrine\ORM\Mapping as ORM;
  
  /**
   * @ORM\MappedSuperclass
   */
  class Base
  {
      /**
       * @ORM\Column(type="integer", nullable=false)
       */
      private $someColumn;
  }
  
  /**
   * @ORM\Entity
   * @ORM\Table(uniqueConstraints={@ORM\UniqueConstraint(name="column_idx", columns={"someColumn"})})
   */
  class Concrete extends Base
  {
  }
  </code></pre>
  
  <p>The problem is at processing of @ORM\Table annotation during schema generation:</p>
  
  <pre><code>[Doctrine\DBAL\Schema\SchemaException]                          
  There is no column with name 'someColumn' on table 'Concrete'.
  </code></pre>
  
  <p>Is there a way to define a unique constraint of a mapped superclass?</p>