API-Platform calculated fields

Greeting.php

<?php

namespace App\Entity;

use ApiPlatform\Core\Annotation\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Serializer\Annotation\Groups;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ApiResource(
 *     collectionOperations={
 *          "get" = {"normalization_context" = {"groups" = "Greeting:Collection:Get"}},
 *     }
 * )
 * @ORM\Entity
 */
class Greeting
{
    /**
     * @var int The entity Id
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     * @Groups("Greeting:Collection:Get")
     */
    private $id;
    
    private $a = 1;
    
    private $b = 2;

    /**
     * @var string A nice person
     *
     * @ORM\Column
     * @Groups("Greeting:Collection:Get")
     */
    public $name = '';

    public function getId(): int
    {
        return $this->id;
    }

    /**
     * @return int sum
     * @Groups("Greeting:Collection:Get") <- MAGIC IS HERE, You can set a group on a method.
     */
    public function getSum()
    {
        return $this->a+$this->b;
    }
}

You'll get as result, the calculated field.

in YAML

# api/config/api_platform/resources.yaml
App\Entity\Greeting:
    attributes:
        normalization_context:
            groups: ['greeting:collection:get']

# api/config/serialization/Greeting.yaml
App\Entity\Greeting:
    attributes:
        id:
            groups: ['greeting:collection:get']
        name:
            groups: ['greeting:collection:get']
        sum:
            groups: ['greeting:collection:get']