Select objects having children in Symfony2/Doctrine using DQL - php

Question

I have an Article and Subcategory entities, I want, using Doctrine DQL, select all the subcategories that have 1 or more articles, I don't want to select empty Subcategories .. How can I do that in one query
Here are my objects:
Article
<?php
namespace Evr\ArticleBundle\Entity;
use Evr\HomeBundle\Entity\ImageThumbnail;
use Symfony\Component\HttpFoundation\File\UploadedFile;
use Symfony\Component\Validator\Constraints as Assert;
use Doctrine\ORM\Mapping as ORM;
/**
* Article
*
* #ORM\Table(name="ev_article")
* #ORM\Entity
* #ORM\HasLifecycleCallbacks
*/
class Article{
/**
* #var integer
*
* #ORM\Column(name="article_id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
*
* #ORM\ManyToOne(targetEntity="Evr\HomeBundle\Entity\Subcategory",inversedBy="articles")
* #ORM\JoinColumn(name="subcategory_id",referencedColumnName="subcategory_id")
*/
private $subcategory;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #var integer
*
* #ORM\Column(name="type", type="integer")
*/
private $type;
/**
* #var text
*
* #ORM\Column(name="content", type="text")
*/
private $content;
/**
* #var text
*
* #ORM\Column(name="exclusive_content", type="text")
*/
private $exclusive_content;
/**
* #var \DateTime
*
* #ORM\Column(name="creation_date", type="date")
*/
private $creation_date;
/**
* #var integer
*
* #ORM\Column(name="views", type="integer" , nullable=true)
*/
private $views;
/**
* #var integer
*
* #ORM\Column(name="votes", type="integer", nullable=true)
*/
private $votes;
/**
* #var string
*
* #ORM\Column(name="photo", type="string", length=255, nullable=true)
*/
protected $photo;
/**
* Image file
*
* #var File
*
* #Assert\File(
* maxSize = "5M",
* mimeTypes = {"image/jpeg", "image/gif", "image/png", "image/tiff"},
* maxSizeMessage = "The maxmimum allowed file size is 5MB.",
* mimeTypesMessage = "Only the filetypes image are allowed."
* )
*/
protected $file;
private $temp;
public function getAbsolutePath() {
return (null === $this->photo) ? null : $this->getUploadRootDir() . '/' . $this->photo;
}
public function getWebPath() {
return (null === $this->photo) ? null : $this->getUploadDir() . '/' . $this->photo;
}
protected function getUploadRootDir() {
return __DIR__ . '/../../../../web/' . $this->getUploadDir();
}
public function getUploadDir() {
return 'uploads/documents/';
}
public function getThumbPath() {
return 'uploads/documents/thumbs/';
}
/**
* Get id
*
* #return integer
*/
public function getId() {
return $this->id;
}
/**
* Set subcategory
*
* #param integer $subcategory
* #return Article
*/
public function setSubcategory($subcategory) {
$this->subcategory = $subcategory;
return $this;
}
/**
* Get subcategory
*
* #return integer
*/
public function getSubcategory() {
return $this->subcategory;
}
/**
* Set title
*
* #param string $title
* #return Article
*/
public function setTitle($title) {
$this->title = $title;
return $this;
}
/**
* Get title
*
* #return string
*/
public function getTitle() {
return $this->title;
}
/**
* Set type
*
* #param integer $type
* #return Article
*/
public function setType($type) {
$this->type = $type;
return $this;
}
/**
* Get type
*
* #return integer
*/
public function getType() {
return $this->type;
}
/**
* Set content
*
* #param text $content
* #return Article
*/
public function setContent($content) {
$this->content = $content;
return $this;
}
/**
* Get content
*
* #return text
*/
public function getContent() {
return $this->content;
}
/**
* Set exclusive_content
*
* #param text $exclusiveContent
* #return Article
*/
public function setExclusiveContent($exclusiveContent) {
$this->exclusive_content = $exclusiveContent;
return $this;
}
/**
* Get exclusive_content
*
* #return text
*/
public function getExclusiveContent() {
return $this->exclusive_content;
}
/**
* Set creation_date
*
* #param DateTime $creationDate
* #return Article
*/
public function setCreationDate($creation_date) {
$this->creation_date = $creation_date;
return $this;
}
/**
* Get creation_date
*
* #return DateTime
*/
public function getCreationDate() {
return $this->creation_date;
}
/**
* Set views
*
* #param integer $views
* #return Article
*/
public function setViews($views = 0) {
$this->views = $views;
return $this;
}
/**
* Get views
*
* #return integer
*/
public function getViews() {
return $this->views;
}
/**
* Set votes
*
* #param integer $votes
* #return Article
*/
public function setVotes($votes) {
$this->votes = $votes;
return $this;
}
/**
* Get votes
*
* #return integer
*/
public function getVotes() {
return $this->votes;
}
/**
* Set photo
*
* #param string $photo
* #return Article
*/
public function setPhoto($photo) {
$this->photo = $photo;
return $this;
}
/**
* Get photo
*
* #return string
*/
public function getPhoto() {
return $this->photo;
}
/**
* Set file
*
* #param UploadedFile $file
* #return Article
*/
public function setFile(UploadedFile $file = null) {
$this->file = $file;
return $this;
}
/**
* Get file
*
* #return UploadedFile
*/
public function getFile() {
return $this->file;
}
/**
* Called before saving the entity
*
* #ORM\PrePersist()
* #ORM\PreUpdate()
*/
public function preUpload() {
if (null !== $this->file) {
$filename = sha1(uniqid(mt_rand(), true));
$this->photo = $filename . '.' . $this->file->guessExtension();
}
}
/**
* Called after entity persistence
*
* #ORM\PostPersist()
* #ORM\PostUpdate()
*/
public function upload() {
if (null === $this->file) {
return;
}
$this->file->move(
$this->getUploadRootDir(), $this->photo
);
$this->file = null;
}
/**
* Called before entity removal
*
* #ORM\PostRemove()
*/
public function removeUpload() {
if ($file = $this->getAbsolutePath()) {
if (file_exists($file)) {
unlink($file);
}
}
}
public function updateUploadedFile($file) {
$this->removeUpload();
$this->file = $file;
$this->preUpload();
$this->upload();
}
}
And Subcategory
<?php
namespace Evr\HomeBundle\Entity;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;
/**
* Subcategory
*
* #ORM\Table(name="ev_subcategory")
* #ORM\Entity
*/
class Subcategory
{
/**
* #var integer
*
* #ORM\Column(name="subcategory_id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
*
* #ORM\ManyToOne(targetEntity="Category",inversedBy="subcategories")
* #ORM\JoinColumn(name="category_id",referencedColumnName="category_id")
*/
private $category;
/**
* #var string
*
* #ORM\Column(name="subcategory", type="string", length=255)
*/
private $subcategory;
/**
* #ORM\OneToMany(targetEntity="Evr\ArticleBundle\Entity\Article", mappedBy="subcategory")
*/
protected $articles;
/**
* #ORM\OneToMany(targetEntity="Evr\CourseBundle\Entity\Course", mappedBy="subcategory")
*/
protected $courses;
public function __construct(){
$this->articles=new ArrayCollection();
$this->courses=new ArrayCollection();
}
/**
* Get id
*
* #return integer
*/
public function getId()
{
return $this->id;
}
/**
* Set category
*
* #param integer $category
* #return Subcategory
*/
public function setCategory($category) {
$this->category = $category;
return $this;
}
/**
* Get category
*
* #return integer
*/
public function getCategory() {
return $this->category;
}
/**
* Set subcategory
*
* #param string $subcategory
* #return Subcategory
*/
public function setSubcategory($subcategory)
{
$this->subcategory = $subcategory;
return $this;
}
/**
* Get subcategory
*
* #return string
*/
public function getSubcategory()
{
return $this->subcategory;
}
/**
* Get articles
*
* #return \Doctrine\Common\Collections\Collection
*/
public function getArticles()
{
return $this->articles;
}
}