How can I serialize only the ID of a child - symfony

Question

I'm looking for the correct way to serialize a child of my object.
I have the following classes:
class company {
/**
* #ORM\Id
* #ORM\GeneratedValue
* #ORM\Column(type="integer")
* #Groups ({"get"})
*/
private $id;
}
class User {
/**
* #ORM\Column(type="string", length=180, unique=true)
* #Groups ({"get"})
*/
private $email;
/**
* #ORM\ManyToOne(targetEntity=Company::class, inversedBy="users")
* #ORM\JoinColumn(nullable=false)
* #Groups ({"get"})
*/
private $company;
}
As soon as I use the serializer on a User object, I receive the following response.
{
"id": 1,
"email": "email#mydomain.com",
"company": {
"id": 1
}
}
But I prefer the following response, how can I get these?
{
"id": 1,
"email": "email#mydomain.com",
"company": 1
}

Answer 1

You can create a custom normalizer, and summarise the company within the User - as it is being converted from the original object (with sub-entities) to an array, before being json-encoded.
symfony console make:serializer:normalizer [optional name, eg: 'UserNormalizer']
This creates a new class, with in part, the contents:
public function normalize($object, $format = null, array $context = []): array
{
// $object is a User entity at this point
$data = $this->normalizer->normalize($object, $format, $context);
// Here: add, edit, or delete some data
// and we summarise the company entity to just the ID.
$data['company'] = $object->getCompany()->getId();
return $data;
}
When I did this with a slightly more complex entity that referred back to the original one (if company had a reference back to a user), it made 'A circular reference', so I added an annotation to #Ignore the field in the User entity, for serialization purposes. It was still given to the normalizer, to use from the object passed into normalize().

Answer 2

You could also serialize the result of a method instead:
class User {
/**
* #ORM\Column(type="string", length=180, unique=true)
* #Groups ({"get"})
*/
private $email;
/**
* #ORM\ManyToOne(targetEntity=Company::class, inversedBy="users")
* #ORM\JoinColumn(nullable=false)
*/
private $company;
/**
* #Groups ({"get"})
*/
public function getCompanyId()
{
return $this->company->getId();
}
}