Doctrine 2 One-To-One unidirectional relationship delete non-owning side

I have two entities with a one-to-one unidirectional relationship:

class Foo {
 ...
/**
 * @OneToOne(targetEntity="Bar")
 */
private $bar;
...
}

class Bar {
...
}

When I try to delete a Bar entity I get this error:

Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails

How do I keep a unidirectional relationship without loosing the ability to delete Bar entities?