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

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.