doctrine nested entities cascade persist : how to reuse existing entities - symfony

Question

If entity A contains multiple entity B and has cascade:persist, how to reuse existing entities B when persisting ?
B entity has one primary key, an integer, and the id of the A parent. The only data it contains is the primary key.
Example:
A has 2 B entities, identified by their id, 14 and 23.
A.Bs = [{id=14, AId=A.id}, {id=23, AId=A.Id}]
Now if I modify this managed entity, to add a B entity to A, with id = 56.
A.Bs = [{id=14, AId=A.id}, {id=23, AId=A.Id}, {id=56}]
Relationships
Entity A
/**
* #var B[]|ArrayCollection
*
* #ORM\OneToMany(targetEntity="B", mappedBy="A", cascade={"persist", "remove"}, orphanRemoval=true)
* #Assert\Valid
*/
private $Bs;
Entity B
/**
* #var A
*
* #ORM\ManyToOne(targetEntity="A", inversedBy="Bs")
* #ORM\JoinColumn(name="A_id", referencedColumnName="A_id")
* #Assert\NotNull()
*/
private $A;
If I try to persist I get Integrity constraint violation, because Doctrine tries to persist the existing entities, that have id 14 and 23.
I understand this is expected behaviour, but how can I make it persist new entities, and reuse existing ones ?
More details:
If I get an existing entity A with $em->find($id) and directly use persist and flush, I will get UniqueConstraintException because it tries to persist the already persisted B entities.
Example code:
/** #var A $existingEntityA */
$existingEntityA = $this->getEntity($id);
$this->serializerFactory->getComplexEntityDeserializer()->deserialize(json_encode($editedEntityADataJson), A::class, 'json', ['object_to_populate' => $existingEntityA]);
$this->entityValidator->validateEntity($existingEntityA);
$this->_em->flush();
Example error : Integrity constraint violation: 1062 Duplicate entry '777111' for key 'PRIMARY'

Answer 1

If I understand your example properly - you're doing something like this:
$b = new B();
$b->setId(56);
$a->getB()->add($b);
and you having a row with primary key 56 into database table that is represented by B?
If my assumption is correct - it is wrong way to go. Reason is that Doctrine internally stores so called "identity map" that keeps track of all entities that either being fetched from database or persisted by calling EntityManager::persist(). Every entity that is scheduled for commit but not available into identity map is considered as "new" and scheduled for insertion. If row with same primary key is already available in database - you're receiving UniqueConstraintException.
Doctrine doesn't handle a case "let me look if there is an entity with such primary key in database" by itself because it will hurt performance significantly and is not needed in most cases. Each such test will result into database query, imagine if you will have thousands of such entities. Since Doctrine doesn't know business logic of your application - it will spend even more resources with attempts to guess optimal strategy so this is intentionally left out of scope.
Correct way for you would be to get your entity by itself before adding to collection:
$newB = $em->find(B::class, 56);
if ($newB) {
$a->getB()->add($newB);
}
In this case new entity will internally have "managed" status and will be correctly handled by Doctrine at a time of commit.