/** @Entity */
class First
{
    /** @OneToMany(targetEntity="Second", mappedBy="parent") */
    protected $secondList;

    // access methods here
    public function __construct()
    {
       $this->secondList = new ArrayCollection();
    }
}

/** @Entity */
class Second
{
    /**
     * @ManyToOne(targetEntity="First", inversedBy="secondList")
     * @JoinColumn(name="First_id", referencedColumnName="Id")
     */
    protected $parent;
}

$first = new Second();
$second = new First();
$first->getSecond()->add($second);
$em->persist($second);
$em->persist($first);

In the case of bi-directional associations you have to update the fields on both sides:

$second->setParent($first);

/** @OneToMany(targetEntity =  "Second", mappedBy = "parent" ) */