public function moveFacetDown(Facet $facet)
    {
        $currentPosition = $facet->getPosition();

        if ($currentPosition > 0) {
            $prevPosition = $currentPosition - 1;
            $prevFacet = $this->om
                ->getRepository('ClarolineCoreBundle:Facet\Facet')
                ->findOneBy(array('position' => $prevPosition));
            $prevFacet->setPosition($currentPosition);
            $facet->setPosition($prevPosition);
            $this->om->persist($prevFacet);
            $this->om->persist($facet);
            $this->om->flush();
        }
    }