How does PersistentCollection influence IdentityMap by using removeElement and remove? - orm

Question

file:Element.php
<?php
use Doctrine\Common\Collections\{
Collection,
ArrayCollection
};
use Doctrine\ORM\Mapping\{
Id,
GeneratedValue,
Table,
Mapping,
Entity,
Column,
JoinColumn,
ManyToOne
};
use Doctrine\DBAL\Types\Types;
#[Entity]
#[Table(name: 'elements')]
class Element
{
#[Id, GeneratedValue, Column(type: Types::INTEGER)]
protected $id;
// OWNER SIDE
#[ManyToOne(targetEntity: \SomeClass::class, inversedBy: 'elementsCollection')]
#[JoinColumn(name: 'some_class_id')]
protected $someClass;
public function getSomeClass()
{
return $this->someClass;
}
public function removeSomeClass()
{
$this->someClass = NULL;
}
}
file: SomeClass.php
<?php
use Doctrine\Common\Collections\{
Collection,
ArrayCollection
};
use Doctrine\ORM\Mapping\{
Id,
GeneratedValue,
Table,
Mapping,
Entity,
Column,
OneToMany
};
use Doctrine\DBAL\Types\Types;
#[Entity]
#[Table(name: 'some_classes')]
class SomeClass
{
#[Id, GeneratedValue, Column(type: Types::INTEGER)]
protected $id;
public function __construct()
{
$this->elementsCollection = new ArrayCollection;
}
// INVERSE SIDE
#[OneToMany(targetEntity: \Element::class, mappedBy: 'someClass')]
protected Collection $elementsCollection;
public function getElementsCollection()
{
return $this->elementsCollection;
}
}
file: index.php
$em = DI::get('mainOEM');
// IdentityMap knows about this $el, because I downloded its.
$el = $em->find(\Element::class, 1);
DUMP
\var_dump($el->getSomeClass()); // object(DoctrineProxies\__CG__\SomeClass)...
$el->removeSomeClass();
DUMP
\var_dump($el->getSomeClass()); // null
SomeClass Entity contains collection of Elements (SomeClass->elementsCollection is inverse side)
$someClassOjb = $em->find(\SomeClass::class, 1);
call PersistentCollection->removeElement($el)
$someClassOjb->getElementsCollection()->removeElement($el);
Now, I just removed $el from the $someClassOjb's collection.
But... $el is updated, it isn't NULL now.
DUMP
// object(DoctrineProxies\__CG__\SomeClass)... like in the beginning
\var_dump($el->getSomeClass());
Ok, IdentityMap was updated by PersistentCollection by getting Elements of SomeClass relationship...
BUT Why PersistentCollection updated $el which I'd already got earlier?!?! I'd already downloaded its from the schema!
How does PersistentCollection update IdentityMap?