Doctrine 2 ManyToMany cascade

Is it possible in Doctrine 2 to create two objects that are many to many related and call persist only on one of them to save both?

User entity:

    /**
 * Owning Side
 *
 * @ManyToMany(targetEntity="Role", inversedBy="users", cascade={"persist"})
 * @JoinTable(name="user_roles",
 *      joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
 *      inverseJoinColumns={@JoinColumn(name="role_id", referencedColumnName="id")}
 *      )
 */
public $roles;

Role entity:

    /**
 * Inverse Side
 *
 * @ManyToMany(targetEntity="User", mappedBy="roles")
 */
public $users;

Saving:

    $role = new Role();

    $user = new User();

$user->roles->add($role);
$role->users->add($user);

$em->persist($user);
$em->flush();

It doesn't work and trows an error "A new entity was found through a relationship that was not configured to cascade persist operations: Entities\Role@0000000004a29c11000000005c48cb75. Explicitly persist the new entity or configure cascading persist operations on the relationship."