Symfony / Twig - Recursion decrease db queries - symfony

Question

I have my category tree in twig template:
{% for category in categories %}
<li>
<div class="li">{{ category.name|trans({}, 'categories')|raw }}</div>
{% if category.children is not empty %}
<ul>
{% include "default/_menu_links.html.twig" with {'categories':category.children} only %}
</ul>
{% endif %}
</li>
{% endfor %}
It creates +- 53 database queries if I have 6 categories and 7 subcategories in each single category.
Is there a way to decrease that number?
I'm using useResultCache(true) in doctrine, but looks like it's not loading from the cache(at least not in dev mode).
How are you handling category trees?
UPDATE:
Entity:
...
/**
* One Category has Many Subcategories.
* #ORM\OneToMany(targetEntity="Category", mappedBy="parent", cascade={"persist"}))
*/
private $children;
/**
* Many Subcategories have One Category.
* #ORM\ManyToOne(targetEntity="Category", inversedBy="children", cascade={"persist"})
* #ORM\JoinColumn(name="parent_id", referencedColumnName="id")
*/
private $parent;
...
/**
* Add child
*
* #param \App\Entity\Product\Category $child
*
* #return Category
*/
public function addChild(\App\Entity\Product\Category $child): Category
{
$this->children[] = $child;
return $this;
}
/**
* Remove child
*
* #param \App\Entity\Product\Category $child
*/
public function removeChild(\App\Entity\Product\Category $child)
{
$this->children->removeElement($child);
}
/**
* Get children
*
* #return \Doctrine\Common\Collections\Collection
*/
public function getChildren(): Collection
{
return $this->children;
}
/**
* Set parent
*
* #param \App\Entity\Product\Category $parent
*
* #return Category
*/
public function setParent(\App\Entity\Product\Category $parent = null): Category
{
$this->parent = $parent;
return $this;
}
/**
* Get parent
*
* #return \App\Entity\Product\Category
*/
public function getParent()
{
return $this->parent;
}
...

Answer 1

According to my comment: you have to JOINyour subcategories. I'm assuming you are currently doing something like this to retrieve your categories:
public function getCategories()
{
return $this->getEntityManager()->createQueryBuilder()
->select("category")
->from("App:Category", "category")
->where("category.parent IS NULL")
->getQuery()->getResult();
}
So if you are now iterating over this array of categories and trying to access the children property, a sub query will be fired for each children which causes this high amount of database queries.
Instead you should JOIN them like this:
public function getCategories()
{
return $this->getEntityManager()->createQueryBuilder()
->select("category", "subcat")
->from("App:Category", "category")
->leftJoin("category.children", "subcat")
->where("category.parent IS NULL")
->getQuery()->getResult();
}
If you now iterate over your categories and access the children property, no extra query will be fired!
Using the query above and this snippet in twig will result only in one database query:
{% for category in categories %}
<p>cat={{ category.id }}</p>
{% for subcat in category.children %}
<p>subcat={{ subcat.id }}</p>
{% endfor %}
<hr>
{% endfor %}