Symfony Parent Child category relation - php

Question

I have a Product Category table, where I have stored categories if parent_id is null then assume this is parent category if not null then it has child categories. Now the issue is I can't show parent category name in listing page, so can someone have an idea how to make a relation in Product Category entity?
Symfony version: 3.3
Table Structure:(product_category)
id
parent_id
slug
title
I have tried this code in ProductCategory entity but it doesn't show any relational data:
class ProductCategory
{
/**
* #ORM\OneToMany(targetEntity="ProductCategory", mappedBy="parent")
* #ORM\JoinColumn(name="parent_id", referencedColumnName="id", nullable=true)
*/
private $parent;

Answer 1

I usually use children and parent and this is how I use It:
/**
* One ProductCategory has Many ProductCategories.
* #ORM\OneToMany(targetEntity="ProductCategory", mappedBy="parent")
*/
private $children;
/**
* Many ProductCategories have One ProductCategory.
* #ORM\ManyToOne(targetEntity="ProductCategory", inversedBy="children")
* #ORM\JoinColumn(name="parent_id", referencedColumnName="id", nullable=true)
*/
private $parent;
EDIT
/**
* ProductCategory constructor.
*/
public function __construct()
{
$this->children = new ArrayCollection();
}
Getters / Setters and this stuff:
/**
* #param ProductCategory $children
* #return $this
*/
public function addChildren (ProductCategory $children)
{
$this->children[] = $children;
return $this;
}

Answer 2

Using entity with self reference, as Alessandro described, then let's say you have all your records extracted with doctrine using $qb->(bla bla bla)->getResult(); into a variable called $categories. Then you just have to iterate through it like that:
foreach($categories as $category) {
echo $category->getTitle() . ' has parent ' . ($category->getParent() ? $category->getParent()->getTitle() : 'none');
}
To understand more about array collection, read: http://www.doctrine-project.org/api/common/2.3/class-Doctrine.Common.Collections.ArrayCollection.html