concert_id stays empty:

		$c = new \Entities\Concert();

                    $pic = new \Entities\Picture();
                    $pic->url     = $img_url;
                    $pic->text    = $img_name;
                    $pic->name    = "$img_name.$img_suff";

		$c->pictures->add($pic);
                $this->em->persist($c);
                $this->em->flush();

		However, this works:

                    $pic = new \Entities\Picture();
                    $pic->concert = $c;
                    $pic->url     = $img_url;
                    $pic->text    = $img_name;
                    $pic->name    = "$img_name.$img_suff";
                    $this->em->persist($pic);
                    $this->em->flush();