Use doctrine in a personalized class

I have a Util class in a Symfony 2.4 proyect that get the slug from a book. The slug cannot be repeated. Each slug must be unique. I have the following code form the class that generate the slug. I have the error:

Fatal error: Using $this when not in object context in C:\xampp\htdocs\gens\src\
Sakya\GensBundle\Util\Util.php on line 38

namespace Sakya\GensBundle\Util;
use Symfony\Component\HttpFoundation\Response;
use Doctrine\ORM\EntityManager;

class Util
{
    protected $em;
    public function __construct(EntityManager $em)
    {
            $this->em = $em;
    }

    public static function getSlug($slug)
    {
           $separador = '-';
           $tildes=array('á','é','í','ó','ú');
           $vocales=array('a','e','i','o','u');
           str_replace($vocales,$tildes,$slug);
           $slug = iconv('UTF-8', 'ASCII//TRANSLIT', $slug);
           $slug = preg_replace("/[^a-zA-Z0-9\/_|+ -]/", '', $slug);
           $slug = strtolower(trim($slug, $separador));
           $slug = preg_replace("/[\/_|+ -]+/", $separador, $slug);




               $i = 1;
               while (! $this->em->getRepository("GensBundle:Libro")->findBySlug($slug)){
                   $slug = $slug."-".$i;
                   $i++;
               }

          return $slug;
       }
}

It is mi Entity "Libro"

<?php

namespace Sakya\GensBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Sakya\GensBundle\Util\Util;
use Symfony\Component\HttpFoundation\File\UploadedFile;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * Libro
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class Libro
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="libro", type="string", length=255)
     */
    private $libro;

    /**
     * @ORM\Column(type="string")
     *
     * @Assert\NotBlank()
     */
    protected $slug;

    /**
     * @var string
     *
     * @ORM\Column(name="autor", type="string", length=255)
     */
    private $autor;

    /**
     * @var string
     *
     * @ORM\Column(name="prefacio", type="string", length=700)
     */
    private $prefacio;

    /**
     * @ORM\OneToMany(targetEntity="Capitulo", mappedBy="libro")
     */
    private $capitulo;

    public function __toString()
    {
        return $this->getLibro();
    }


    /**
     * Constructor
     */
    public function __construct()
    {
        $this->capitulo = new \Doctrine\Common\Collections\ArrayCollection();
    }

    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set libro
     *
     * @param string $libro
     * @return Libro
     */
    public function setLibro($libro)
    {
        $this->libro = $libro;
        $this->slug = Util::getSlug($libro);

        return $this;
    }

    /**
     * Get libro
     *
     * @return string 
     */
    public function getLibro()
    {
        return $this->libro;
    }

    /**
     * Set autor
     *
     * @param string $autor
     * @return Libro
     */
    public function setAutor($autor)
    {
        $this->autor = $autor;

        return $this;
    }

    /**
     * Get autor
     *
     * @return string 
     */
    public function getAutor()
    {
        return $this->autor;
    }

    /**
     * Add capitulo
     *
     * @param \Sakya\GensBundle\Entity\Capitulo $capitulo
     * @return Libro
     */
    public function addCapitulo(\Sakya\GensBundle\Entity\Capitulo $capitulo)
    {
        $this->capitulo[] = $capitulo;

        return $this;
    }

    /**
     * Remove capitulo
     *
     * @param \Sakya\GensBundle\Entity\Capitulo $capitulo
     */
    public function removeCapitulo(\Sakya\GensBundle\Entity\Capitulo $capitulo)
    {
        $this->capitulo->removeElement($capitulo);
    }

    /**
     * Get capitulo
     *
     * @return \Doctrine\Common\Collections\Collection 
     */
    public function getCapitulo()
    {
        return $this->capitulo;
    }

    /**
     * Set prefacio
     *
     * @param string $prefacio
     * @return Libro
     */
    public function setPrefacio($prefacio)
    {
        $this->prefacio = $prefacio;

        return $this;
    }

    /**
     * Get prefacio
     *
     * @return string 
     */
    public function getPrefacio()
    {
        return $this->prefacio;
    }

    /**
     * Set slug
     *
     * @param string $slug
     * @return Libro
     */
    public function setSlug($slug)
    {
        $this->slug = $slug;

        return $this;
    }

    /**
     * Get slug
     *
     * @return string 
     */
    public function getSlug()
    {
        return $this->slug;
    }
}

How can I make a consult that check that the slug is not in the database form the Uti Class?

Thanks for all and sorry for my English

The problem is you are trying refer to a property from static method. Don't do it unless the property is also static.

The best way I can suggest is change the static method into not-static, then create a service from Util class, and make validation outside the entity. It's not a good idea using service inside entity class.

You didn't show the context but I can guess: if you want set slug based on a form - then the best way to achieve what you want would be create a validator.