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// ...

$discriminator = new ClassDiscriminatorResolver();
// $type is the property used to differentiate between classes
$discriminator->addClassMapping(
    CodeRepository::class,
    new ClassDiscriminatorMapping('type', [
        'github' => GitHubCodeRepository::class,
        'bitbucket' => BitBucketCodeRepository::class,
    ])
);

// pass the discriminator as the last argument of the normalizer
$serializer = new Serializer(
    array(new ObjectNormalizer(null, null, null, null, $discriminator)),
    array('json' => new JsonEncoder())
);

// when serializing, the discriminator knows that the value of the "type"
// property for GitHubCodeRepository classes must be set to {"type": "github"}
$serialized = $serializer->serialize(new GitHubCodeRepository());

// when deserializing into the abstract CodeRepository class, the discriminator
// uses the "type" value to know that $repository must be an instance of GitHubCodeRepository
$repository = $serializer->unserialize($serialized, CodeRepository::class, 'json');

namespace App\Repository;

use Symfony\Component\Serializer\Annotation\DiscriminatorMap;

/**
 * @DiscriminatorMap(typeProperty="type", mapping={
 *    "github"="App\Repository\GitHubCodeRepository",
 *    "bitbucket"="App\Repository\BitBucketCodeRepository"
 * })
 */
interface CodeRepository
{
    // ...
}