Symfony2-Doctrine: ManyToMany relation is not saved to database

Your Category entity is the inverse side of the relationship.

Try changing addItems to look like this:

public function addItem(\Ako\StoreBundle\Entity\Item $item)
    {
        $item->addCategory($this);
        $this->items[] = $item;
    }

Note that I changed your plural names to singular, since you’re dealing with single entities, not collections.