/**
 * @Entity
 */
class A
{
    /**
     * @OneToMany(targetEntity="B", mappedBy="a", cascade={"PERSIST"})
     */
    protected $bCollection;

    public function addB(B $b)
    {
        // update both sides of the relationship
        $b->setA($this);
        $this->bCollection->add($b);
    }
}

/**
 * @Entity
 */
class B
{
    /**
     * @ManyToOne(targetEntity="A", inversedBy="bCollection")
     */
    protected $a;

    public function setA(A $a)
    {
        $this->a = $a;
    }
}


// Usage
$a = new A();
$b1 = new B();
$b2 = new B();

$a->addB($b1);
$a->addB($b2);

$em->persist($a);
/* Without the cascade={"PERSIST"} we have to do this
$em->persist($b1);
$em->persist($b2);
 */