In my web application, which is built with Symfony2, contains the following entities:

```
/**
 * @ORM\Entity
 * @ORM\Table
 */
class Entity
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     * @ORM\Column(name="id", type="integer")
     */
    private $id;

    /**
     * @ORM\OneToMany(targetEntity="MappedSuperclass", mappedBy="entity", cascade={"persist", "remove"})
     */
    private $mappedSuperclasses;
}

/**
 * @ORM\MappedSuperclass
 */
abstract class MappedSuperclass
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     * @ORM\Column(name="id", type="integer")
     */
    protected $id;

    /**
     * @ORM\ManyToOne(targetEntity="Entity", inversedBy="mappedSuperclasses")
     * @ORM\JoinColumn(name="entity_id", referencedColumnName="id", nullable=false)
     */
    protected $entity;
}

/**
 * @ORM\Entity
 * @ORM\Table(name="table_1")
 */
class Subclass1 extends MappedSuperclass
{
    /**
     * @ORM\Column(name="unique_member", type="string")
     */
    private $uniqueMember;
}

/**
 * @ORM\Entity
 * @ORM\Table(name="table_2")
 */
class Subclass2 extends MappedSuperclass
{
    /**
     * @ORM\Column(name="unique_member", type="string")
     */
    private $uniqueMember; // This is different from Subclass1
}

```

I'll explain this a bit. An `Entity` has a collection of `MappedSuperclass`. `MappedSuperclass` is an abstract class which contains some common variables for its subclasses. `Subclass1` and `Subclass2` are subclasses of `MappedSuperclass`. When an `Entity` is removed from database, the items in `$mappedSuperclasses` should be removed together, that's why the `cascade={"persist", "remove"}` is set.

However, when I try to delete an `Entity`, I got the following error:

`ContextErrorException: Notice: Undefined index: entity in C:\project\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\BasicEntityPersister.php line 1753`

If I change the `targetEntity` of `Entity::$mappedSuperclasses` to `Subclass1` or `Subclass2`, it will work. Is my set up impossible to achieve `ON DELETE CASCADE`? What am I missing?