Symfony2 DQL How to join the last row in a OneToMany relation - symfony

Question

I have two entities related by a OneToMany relation:
<?php
namespace CRMBundle\Entity;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
/**
* User
*
* #ORM\Table()
* #ORM\Entity(repositoryClass="CRMBundle\Entity\ContactRepository")
*/
class User
{
/*...*/
/**
* #ORM\OneToMany(targetEntity="CRMBundle\Entity\Message", mappedBy="user", cascade={"persist"})
* #ORM\OrderBy({"datetime" = "DESC"})
*/
protected $messages;
/*...*/
}
And
<?php
namespace CRMBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
/**
* Message
*
* #ORM\Table()
* #ORM\Entity
*/
class Message
{
/*...*/
/**
* #ORM\ManyToOne(targetEntity="CRMBundle\Entity\User", inversedBy="messages")
* #ORM\JoinColumn(name="user_id", referencedColumnName="id", onDelete="SET NULL")
*/
private $user;
/**
* #var \DateTime
*
* #ORM\Column(name="Datetime", type="datetime", nullable=true)
*/
private $datetime;
/*...*/
}
My question is how to create a query in the UserController to get every user with the last message (i.e. the most recent according to the datetime attribute) of each user?

Answer 1

I think what you are looking for is in one of my previous answers to one of my own questions ^^
You have to use a subquery to select dynamically the most recent datetime value of one user's messages, and to join the message having this value.
To do this, you must define the (sub) query selecting the MAX value of message.datetime:
$qb2= $this->createQueryBuilder('ms')
->select('MAX(ms.datetime) maxDate')
->where('ms.user = u')
;
And then use it in your join clause, the whole function being in your UserRepository:
$qb = $this->createQueryBuilder('u');
$qb ->leftJoin('u.messages', 'm', 'WITH', $qb->expr()->eq( 'm.datetime', '('.$qb2->getDQL().')' ))
->addSelect('m');
Your User (each of them) will then have a messages Collection, containing one (or null if no message from the user) message, which you will get this way:
$user->getMessages()->first();
But if you use the lazy loading function of Symfony, as you have already defined an orderby annotation on your user.messages attribute, calling
$user->getMessages()->first()
should return to you the most recent message (but will also load all the others silently).
To avoid this silent second DB query, you can join it directly to the query requesting your users:
$qb = $this->createQueryBuilder('u');
$qb ->leftJoin('u.messages', 'm')
->addSelect('m');