How to get the newest entity in a OneToMany relation with Doctrine - symfony

Question

Intro
For this question the following two entities are the main characters:
ObjectElementTask
Activitites
One ObjectElementTask can have many activities.
Goal
I want to get a collection of ALL Activities and order them by ObjectElementTask and get only the last Activity (based on the 'executiondate' of the activities).
Question
How can i setup the querybuilder from Doctrine to achieve my goal? BTW: I am using Symfony 2.8.
Entities
ObjectElementTask:
class ObjectElementTask
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\OneToMany(targetEntity="Activity", mappedBy="objectelementtask", cascade={"persist", "remove"}, orphanRemoval=true)
* #ORM\OrderBy({"executionDate" = "DESC"})
* #Expose
*/
private $activities;
Activity
class Activity {
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\ManyToOne(targetEntity="ObjectElementTask", inversedBy="activities", cascade={"persist"})
* #ORM\JoinColumn(name="objectelementtask_id", referencedColumnName="id", onDelete="CASCADE")
*/
private $objectelementtask;
/**
* #ORM\Column(type="datetime", nullable=true)
*/
protected $executionDate;
Thanks in advance!
Edit
Based on the answer in this question:
DQL Select every rows having one column's MAX value
i have come up with the following code:
$qb = $this->getDoctrine()
->getRepository('AppBundle:Activity')
->createQueryBuilder('a')
->select('MAX(a.executionDate) maxDate');
$qb2 = $this->getDoctrine()
->getRepository('AppBundle:ObjectElementTask')
->createQueryBuilder('o');
$qb2->innerJoin('AppBundle:ObjectElementTask', 'oe', 'WITH', $qb2->expr()->eq('oe.activities', '(' . $qb->getDQL() . ')'));
But as you can guess, this doesn't work. It only returns one row (the newest in the Activity). And i want the newest activity of each ObjectElementTask.
Edit 2
I tried this code:
$activities = $this->getDoctrine()
->getRepository('AppBundle:Activity')
->createQueryBuilder('a')
->select('a, MAX(a.executionDate) AS HIDDEN max_date')
->groupBy('a.objectelementtask')
->getQuery()
->getResult();
And the max date returns the value of the last activitie but not the full entity (only the max date...)
Edit 3:
This is the final solution:
This is how i finally got what i needed:
$em = $this->getEntityManager();
$rsm = new ResultSetMappingBuilder($em);
$query = $em->createNativeQuery('SELECT a1.* FROM activity a1 INNER JOIN ( SELECT id, max(execution_date) MaxPostDate, objectelementtask_id FROM activity GROUP BY objectelementtask_id ) a2 ON a1.execution_date = a2.MaxPostDate AND a1.objectelementtask_id = a2.objectelementtask_id order by a1.execution_date desc', $rsm);
$rsm->addRootEntityFromClassMetadata('AppBundle:Activity', 'a1');
$activities = $query->getResult();

Answer 1

This is how i finally got what i needed:
$em = $this->getEntityManager();
$rsm = new ResultSetMappingBuilder($em);
$query = $em->createNativeQuery('SELECT a1.* FROM activity a1 INNER JOIN ( SELECT id, max(execution_date) MaxPostDate, objectelementtask_id FROM activity GROUP BY objectelementtask_id ) a2 ON a1.execution_date = a2.MaxPostDate AND a1.objectelementtask_id = a2.objectelementtask_id order by a1.execution_date desc', $rsm);
$rsm->addRootEntityFromClassMetadata('AppBundle:Activity', 'a1');
$activities = $query->getResult();