Product.php

<?php

//... Product class code ...

public function addTags(\Khepin\ProductBundle\Entity\Tag $tags)
{
    $this->tags[] = $tags;
    $tags->setProduct($this);
}

public function setTags(array $tags){
    $this->tags = new \Doctrine\Common\Collections\ArrayCollection($tags);
    foreach ($tags as $tag){
        $tag->setProduct($this);
    }
}