/**
 * @ORM\Table(
 *      name="`category`",
 *      indexes={
 *          @ORM\Index(
 *              name="fk_category_parentId",
 *              columns={"parentId"}
 *          )
 *      },
 *      uniqueConstraints={
 *          @ORM\UniqueConstraint(
 *              name="uq_category_nameParentId",
 *              columns={"name", "parentId"}
 *          )
 *      }
 * )
 */
class Category
{
    /**
     * @ORM\Id
     * @ORM\Column(type="smallint", name="`id`")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\Column(type="smallint", name="`parentId`", nullable=true)
     */
    protected $parentId;

    /**
     * @ORM\Column(type="string", length=50, name="`name`")
     */
    protected $name;

    /**
     * @ORM\ManyToOne(targetEntity="Category", inversedBy="categories")
     * @ORM\JoinColumn(name="parentId", referencedColumnName="id", onDelete="CASCADE", onUpdate="NO ACTION")
     */
    protected $category;
}

... 


public function load(ObjectManager $manager)
    {
        $parent = new Category();
        $parent->setName('parent');
        $manager->persist($parent);
        $manager->flush();

        $child = new Category();
        $child->setName('parent');
        $child->setParentId($parent->getId());
        $child->setCategory($parent);

        $manager->persist($child);
        $manager->flush();
}