Symfony OneToMany - ManyToOne entity association not working - symfony

Question

I'm training myself on Symfony and struggling with a problem with bidirectional association (very basic) because by dumping my entity in a twig template I verify that data is correct but the association is always null.
My problem is like this one but the solution is not shared.
I read the documentation here and it seems I follow the right steps.
My db contain a Parent table and a Children table related by children.parent_id as foreign key, both table are popolated and I use DOCTRINE:GENERATE:ENTITIES and DOCTRINE:GENERATE:CRUD.
In Parents class I have:
function __construct() {
$this->lastUpd = new \DateTime();
$this->children = new ArrayCollection();
}
/*
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Children", mappedBy="parent_id", cascade={"persist"})
*/
private $children;
public function setChildren(ArrayCollection $children) {
return $this->children = $children;
}
public function getChildren() {
return $this->children;
}
In Children class I have:
/**
* #var \AppBundle\Entity\Parents
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Parents", inversedBy="children")
* #ORM\JoinColumns({
* #ORM\JoinColumn(name="parent_id", referencedColumnName="parent_id")
* })
*/
private $parent_id;
/**
* Set parent_id
* #param \AppBundle\Entity\Parents $parent_id
* #return Parents
*/
public function setParentID(\AppBundle\Entity\Parents $parent_id= null) {
$this->parent_id = $parent_id;
return $this;
}
/**
* Get parent_id
* #return \AppBundle\Entity\Parents
*/
public function getParentID() {
return $this->parent_id;
}
As additional info looking at Simfony profiler (of parents list page) -> Doctrine -> Entities Mapping I found (with no errors) AppBundle\Entity\Parents and AppBundle\Entity\Type (a working unidirectional OneToMany association).
I am sorry to post a so basic error and I bet the solution is simple but I can't see it.

Answer 1

note: Im assuming that youre not creating an ArrayCollection of children and adding them en'mass.
you dont have any addChild method (which you need to call).
this is easy with an ArrayCollection.
public function addChild(Children $child) {
$this->children->add($child);
}
you could also do with a removeChild as well.
public function removeChild(Children $child) {
$this->children->removeElement($child);
}
then when in your controller.
$child = new Children();
$parent->addChild($child);
then when you persist the parent object, the children will follow due to the cascade persist. I would also add cascade={"remove"} as well, so when you delete the parent, the children will go to.