Symfony Doctrine : get field value in foreach loop on fetchAll() - symfony

Question

I'm trying to display some fields of a table 'Post' using a raw query :
$connection = $em->getConnection();
$statement = $connection->prepare("SELECT * FROM Post WHERE category_id = 1");
$statement->execute();
$posts = $statement->fetchAll();
...
foreach($posts as $post) {
$xml .= $post->getId();
$xml .= $post->getTitle();
$xml .= $post->getContent();
}
I've got an error "FatalErrorException: Error: Call to a member function getId() on a non-object in ..."
All those getters are right in my Post entity. Any suggestion about what I'm doing wrong ?
[EDIT]
$em = $this->getDoctrine()->getManager();
$post_repository = $em->getRepository('MyBundle:Post');
$posts = $post_repository->findBy(array('category_id' => 1));
foreach($posts as $post) {
$xml .= $post->getTitle();
}
Returns me "Unrecognized field: category_id".
My Post class :
class Post
{
/**
* #ORM\ManyToOne(targetEntity="MyBundle\Entity\Category", inversedBy="post")
* #ORM\JoinColumns({
* #ORM\JoinColumn(name="category_id", referencedColumnName="id")
* })
*/
private $category;
/**
* Set category
*
#param MyBundle\Entity\Category $category
*/
public function setCategory(\MyBundle\Entity\Category $category)
{
$this->category = $category;
}
/**
* Get category
*
#return MyBundle\Entity\Category
*/
public function getCategory()
{
return $this->category;
}
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
....

Answer 1

Why using directly your connection here? You should consider using the entity repository for your "posts" class. For example :
$posts = $em->getRepository('YourBundle:Post')->findBy(array('category_id' => 1));
this should work, just replace the YourBundle:Post with the proper bundle and class names. Same for the category_id, I can't guess without your implementation if it's the class property or the mapping name.
I suggest you to read more on the official Doctrine documentation to improve your knowledge on the subject.

Answer 2

When you execute a raw query using Doctrine's DBAL layer the results come back as an array of field names mapped to values rather than as entities.
Therefore you need something like this:
foreach($posts as $post) {
$xml .= $post['id'];
$xml .= $post['title'];
$xml .= $post['content';
}