title: Doctrine @ORM\InheritanceType("SINGLE_TABLE") and empty DiscriminatorMap value
body: <p>i try setup reference with discriminator but have error when i try get subdomain for miejsce - mysql update work fine</p>
  
  <p>error is : 
      An exception occurred while executing 'SELECT t0.id AS id1, t0.name AS name2, t0.rid AS rid3, t0.rid AS rid4, t0.ridType FROM mSubdomains t0 WHERE t0.rid = ? AND t0.ridType IN ()' with params ["5922"]:</p>
  
  <p>is normal to params is only miejsce.id ? ridType should by 1</p>
  
  <p>when i push this query to phpmyadmin i get : </p>
  
  <pre><code>SELECT t0.id AS id1, t0.name AS name2, t0.rid AS rid3, t0.rid AS rid4, t0.ridType
  FROM mSubdomains t0
  WHERE t0.rid =5922
  AND t0.ridType
  IN ( 1 )
  LIMIT 0 , 30
  </code></pre>
  
  <p>and result : </p>
  
  <pre><code>id1     name2   rid3    rid4    ridType     
  1695    test    5922    5922    1
  </code></pre>
  
  <p>strange is rid3 , rid4  ?</p>
  
  <pre><code>class Miejsce
  {
  ...
     /**
       * @ORM\OneToMany(targetEntity="Miejsce\DomainBundle\Entity\MiejsceSubdomain", mappedBy="miejsce", cascade={"all"})
       * @var Subdomain
       */
      protected $subdomain;
  </code></pre>
  
  <p>if MiejsceSubdomain is not abstract i get error  :</p>
  
  <p>MappingException: Entity 'Miejsce\DomainBundle\Entity\MiejsceSubdomain' has to be part of the discriminator map of 'Miejsce\DomainBundle\Entity\Subdomain' to be properly mapped in the inheritance hierachy. Alternatively you can make 'Miejsce\DomainBundle\Entity\Miej`sceSubdomain' an abstract class to avoid this exception from occuring.</p>
  
  <pre><code>/**
   * @ORM\Entity
   */
  abstract class MiejsceSubdomain extends Subdomain
  {
  
      /**
       * @ORM\ManyToOne(targetEntity="Miejsce\ObiektyBundle\Entity\Miejsce")
       * @ORM\JoinColumn(name="rid", referencedColumnName="id")
       */
      protected $miejsce;
  
  }
  
  
  
  /**
   * Subdomain
   * @ORM\Entity
   * @ORM\InheritanceType("SINGLE_TABLE")
   * @ORM\DiscriminatorColumn(name="ridType", type="integer")
   * @ORM\DiscriminatorMap({"1" = "Miejsce\ObiektyBundle\Entity\Miejsce"})
   *
   * @ORM\Table(name="mSubdomains", indexes={
      @ORM\Index(name="name", columns={"name"})
      ,@ORM\Index(name="ridridType", columns={"rid","ridType"})
  }))
   */
  class Subdomain
  {
  
  
      /**
       * @var integer
       * @ORM\Column(type="integer")
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      private $id;
  
      /**
       * @ORM\Column(type="string")
       * @var string
       */
      private $name;
  
      /**
       * @ORM\Column(type="integer")
       * @var integer
       */
      private $rid;
  </code></pre>