The Serialization Process

# Overall Process

API Platform embraces and extends the Symfony Serializer Component to transform PHP entities in (hypermedia) API responses.

Watch the Serializer screencast

The main serialization process has two stages:

Serializer workflow

As you can see in the picture above, an array is used as a man-in-the-middle. This way, Encoders will only deal with turning specific formats into arrays and vice versa. The same way, Normalizers will deal with turning specific objects into arrays and vice versa. – The Symfony documentation

Unlike Symfony or Laravel themselves, API Platform leverages custom normalizers, its router and the state provider system to perform an advanced transformation. Metadata are added to the generated document including links, type information, pagination data or available filters.

The API Platform Serializer is extendable. You can register custom normalizers and encoders in order to support other formats. You can also decorate existing normalizers to customize their behaviors.

# Available Serializers

JSON-LD serializer api_platform.jsonld.normalizer.item

JSON-LD, or JavaScript Object Notation for Linked Data, is a method of encoding Linked Data using JSON. It is a World Wide Web Consortium Recommendation.

HAL serializer api_platform.hal.normalizer.item
JSON, XML, CSV, YAML serializer (using the Symfony serializer) api_platform.serializer.normalizer.item

# The Serialization Context, Groups and Relations

Watch the Serialization Groups screencast

API Platform allows you to specify the $context variable used by the Symfony Serializer. This variable is an associative array that has a handy groups key allowing you to choose which attributes of the resource are exposed during the normalization (read) and denormalization (write) processes. It relies on the serialization (and deserialization) groups feature of the Symfony Serializer component.

In addition to groups, you can use any option supported by the Symfony Serializer. For example, you can use enable_max_depth to limit the serialization depth.

# Configuration for Symfony

Just like other Symfony and API Platform components, the Serializer component can be configured using attributes, XML or YAML. Since attributes are easy to understand, we will use them in the following examples.

Note: if you aren’t using the API Platform distribution, you will need to enable annotation support in the serializer configuration:

# api/config/packages/framework.yaml
framework:
  serializer: { enable_annotations: true }

If you use Symfony Flex, just execute composer req doctrine/annotations and you are all set!

If you want to use YAML or XML, please add the mapping path in the serializer configuration:

# api/config/packages/framework.yaml
framework:
  serializer:
    mapping:
      paths: ['%kernel.project_dir%/config/serialization']

# Using Serialization Groups

It is simple to specify what groups to use in the API system:

Add the normalization context and denormalization context attributes to the resource, and specify which groups to use. Here you see that we add read and write, respectively. You can use any group names you wish.
Apply the groups to properties in the object.

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(
    normalizationContext: ['groups' => ['read']],
    denormalizationContext: ['groups' => ['write']],
)]
class Book
{
    #[Groups(['read', 'write'])]
    public $name;

    #[Groups('write')]
    public $author;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources.yaml
resources:
  App\ApiResource\Book:
    normalizationContext:
      groups: ['read']
    denormalizationContext:
      groups: ['write']

# The YAML syntax is only supported for Symfony
# api/config/serialization/Book.yaml
App\ApiResource\Book:
  attributes:
    name:
      groups: ['read', 'write']
    author:
      groups: ['write']

<!--The XML syntax is only supported for Symfony -->
<!-- api/config/api_platform/resources.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<resources xmlns="https://api-platform.com/schema/metadata/resources-3.0"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="https://api-platform.com/schema/metadata/resources-3.0
                               https://api-platform.com/schema/metadata/resources-3.0.xsd">
    <resource class="App\ApiResource\Book">
        <normalizationContext>
            <values>
                <value name="groups">
                    <values>
                        <value>read</value>
                    </values>
                </value>
            </values>
        </normalizationContext>
        <denormalizationContext>
            <values>
                <value name="groups">
                    <values>
                        <value>write</value>
                    </values>
                </value>
            </values>
        </denormalizationContext>
    </resource>
</resources>
    
<!--The XML syntax is only supported for Symfony -->
<!-- api/config/serialization/Book.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<serializer xmlns="http://symfony.com/schema/dic/serializer-mapping"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://symfony.com/schema/dic/serializer-mapping
                                http://symfony.com/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd">
    <class name="App\ApiResource\Book">
        <attribute name="name">
            <group>read</group>
            <group>write</group>
        </attribute>
        <attribute name="author">
            <group>write</group>
        </attribute>
    </class>
</serializer>

In the previous example, the name property will be visible when reading (GET) the object, and it will also be available to write (PUT / PATCH / POST). The author property will be write-only; it will not be visible when serialized responses are returned by the API.

Internally, API Platform passes the value of the normalizationContext as the 3rd argument of the Serializer::serialize() method during the normalization process. denormalizationContext is passed as the 4th argument of the Serializer::deserialize() method during denormalization (writing).

To configure the serialization groups of classes’s properties, you must use directly the Symfony Serializer’s configuration files or attributes.

In addition to the groups key, you can configure any Symfony Serializer option through the $context parameter (e.g. the enable_max_depthkey when using the @MaxDepth annotation).

Any serialization and deserialization group that you specify will also be leveraged by the built-in actions and the Hydra documentation generator.

# Using Serialization Groups per Operation

Watch the Relations screencast

By default, the serializer provided with API Platform represents relations between objects using dereferenceable IRIs. They allow you to retrieve details for related objects by issuing extra HTTP requests. However, for performance reasons, it is sometimes preferable to avoid forcing the client to issue extra HTTP requests.

It is possible to specify normalization and denormalization contexts (as well as any other attribute) on a per-operation basis. API Platform will always use the most specific definition. For instance, if normalization groups are set both at the resource level and at the operation level, the configuration set at the operation level will be used and the resource level ignored.

In the following example we use different serialization groups for the GET and PATCH operations:

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Metadata\Get;
use ApiPlatform\Metadata\Patch;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(normalizationContext: ['groups' => ['get']])]
#[Get]
#[Patch(normalizationContext: ['groups' => ['patch']])]
class Book
{
    #[Groups(['get', 'patch'])]
    public $name;

    #[Groups('get')]
    public $author;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Book.yaml
App\ApiResource\Book:
    normalizationContext:
        groups: ['get']
    operations:
        ApiPlatform\Metadata\Get: ~
        ApiPlatform\Metadata\Get: ~
        ApiPlatform\Metadata\Patch:
            normalizationContext:
                groups: ['patch']

# The YAML syntax is only supported for Symfony
# api/config/serializer/Book.yaml
App\ApiResource\Book:
    attributes:
        name:
            groups: ['get', 'patch']
        author:
            groups: ['get']

<!-- The XML syntax is only supported for Symfony -->
<!-- api/config/api_platform/resources.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<resources xmlns="https://api-platform.com/schema/metadata/resources-3.0"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="https://api-platform.com/schema/metadata/resources-3.0
                               https://api-platform.com/schema/metadata/resources-3.0.xsd">
    <resource class="App\ApiResource\Book">
        <normalizationContext>
            <values>
                <value name="groups">
                    <values>
                        <value>get</value>
                    </values>
                </value>
            </values>
        </normalizationContext>
        <operations>
            <operation class="ApiPlatform\Metadata\Get" />
            <operation class="ApiPlatform\Metadata\Patch">
                <normalizationContext>
            <values>
                <value name="groups">
                    <values>
                        <value>patch</value>
                    </values>
                </value>
            </values>
        </normalizationContext>
            </operation>
        </operations>
    </resource>
</resources>

<!-- The XML syntax is only supported for Symfony -->
<!-- api/config/serialization/Book.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<serializer xmlns="http://symfony.com/schema/dic/serializer-mapping"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://symfony.com/schema/dic/serializer-mapping
                                http://symfony.com/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd">
    <class name="App\ApiResource\Book">
        <attribute name="name">
            <group>get</group>
            <group>patch</group>
        </attribute>
        <attribute name="author">
            <group>get</group>
        </attribute>
    </class>
</serializer>

The name and author properties will be included in the document generated during a GET operation because the configuration defined at the resource level is inherited. However the document generated when a PATCH request will be received will only include the name property because of the specific configuration for this operation.

Refer to the operations documentation to learn more.

# Embedding Relations

Watch the Relations screencast

Note: We strongly recommend using Vulcain instead of this feature. Vulcain allows creating faster (better hit rate) and better designed APIs than relying on compound documents, and is supported out of the box in the API Platform distribution.

# Normalization

In the following JSON document, the relation from a book to an author is by default represented by an URI:

{
  "@context": "/contexts/Book",
  "@id": "/books/62",
  "@type": "Book",
  "name": "My awesome book",
  "author": "/people/59"
}

It is possible to embed related objects (in their entirety, or only some of their properties) directly in the parent response through the use of serialization groups. By using the following serialization groups attributes (#[Groups]), a JSON representation of the author is embedded in the book response. As soon as any of the author’s attributes is in the book group, the author will be embedded.

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(normalizationContext: ['groups' => ['book']])]
class Book
{
    #[Groups('book')]
    public $name;

    #[Groups('book')]
    public $author;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Book.yaml
App\ApiResource\Book:
    normalizationContext:
        groups: ['book']

# The YAML syntax is only supported for Symfony
# api/config/serializer/Book.yaml
App\ApiResource\Book:
    attributes:
        name:
            groups: ['book']
        author:
            groups: ['book']

<?php
// api/src/ApiResource/Person.php with Symfony or app/ApiResource/Person.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource]
class Person
{
    #[Groups('book')]
    public $name;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/serializer/Person.yaml
App\ApiResource\Person:
  attributes:
    name:
      groups: ['book']

The generated JSON using previous settings is below:

{
  "@context": "/contexts/Book",
  "@id": "/books/62",
  "@type": "Book",
  "name": "My awesome book",
  "author": {
    "@id": "/people/59",
    "@type": "Person",
    "name": "Kévin Dunglas"
  }
}

In order to optimize such embedded relations, the default Doctrine state provider will automatically join entities on relations marked as EAGER. This avoids the need for extra queries to be executed when serializing the related objects.

Instead of embedding relations in the main HTTP response, you may want to “push” them to the client using HTTP/2 server push.

# Denormalization

It is also possible to embed a relation in PUT, PATCH and POST requests. To enable that feature, set the serialization groups the same way as normalization. For example:

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;

#[ApiResource(denormalizationContext: ['groups' => ['book']])]
class Book
{
    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Book.yaml
App\ApiResource\Book:
  denormalizationContext:
    groups: ['book']

The following rules apply when denormalizing embedded relations:

If an @id key is present in the embedded resource, then the object corresponding to the given URI will be retrieved through the state provider. Any changes in the embedded relation will also be applied to that object.
If no @id key exists, a new object will be created containing state provided in the embedded JSON document.

You can specify as many embedded relation levels as you want.

# Force IRI with relations of the same type (parent/childs relations)

It is a common problem to have entities that reference other entities of the same type:

<?php
// api/src/ApiResource/Person.php with Symfony or app/ApiResource/Person.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(
    normalizationContext: ['groups' => ['person']],
    denormalizationContext: ['groups' => ['person']]
)]
class Person
{
    #[Groups('person')]
    public $name;

   /**
    * @var Person
    */
    #[Groups('person')]
   public $parent;  // Note that a Person instance has a relation with another Person.

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Person.yaml
App\ApiResource\Person:
    normalizationContext:
        groups: ['person']
    denormalizationContext:
        groups: ['person']

# The YAML syntax is only supported for Symfony
# api/config/serializer/Person.yaml
App\ApiResource\Person:
    attributes:
        name:
            groups: ['person']
        parent:
            groups: ['person']

The problem here is that the $parent property become automatically an embedded object. Besides, the property won’t be shown on the OpenAPI view.

To force the $parent property to be used as an IRI, add an #[ApiProperty(readableLink: false, writableLink: false)] annotation:

<?php
// api/src/ApiResource/Person.php with Symfony or app/ApiResource/Person.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiProperty;
use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(
    normalizationContext: ['groups' => ['person']],
    denormalizationContext: ['groups' => ['person']]
)]
class Person
{
    #[Groups('person')]
    public string $name;

   #[Groups('person')]
   #[ApiProperty(readableLink: false, writableLink: false)]
   public Person $parent;  // This property is now serialized/deserialized as an IRI.

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Person.yaml
resources:
  App\ApiResource\Person:
    normalizationContext:
      groups: ['person']
    denormalizationContext:
      groups: ['person']
properties:
  App\ApiResource\Person:
    parent:
      readableLink: false
      writableLink: false

# The YAML syntax is only supported for Symfony
# api/config/serializer/Person.yaml
App\ApiResource\Person:
  attributes:
    name:
      groups: ['person']
    parent:
      groups: ['person']

# Plain Identifiers for Symfony

Instead of sending an IRI to set a relation, you may want to send a plain identifier. To do so, you must create your own denormalizer:

<?php
// api/src/Serializer/PlainIdentifierDenormalizer

namespace App\Serializer;

use ApiPlatform\Api\IriConverterInterface;
use App\ApiResource\Dummy;
use App\ApiResource\RelatedDummy;
use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
use Symfony\Component\Serializer\Normalizer\DenormalizerAwareInterface;
use Symfony\Component\Serializer\Normalizer\DenormalizerAwareTrait;

class PlainIdentifierDenormalizer implements DenormalizerInterface, DenormalizerAwareInterface
{
    use DenormalizerAwareTrait;

    private $iriConverter;

    public function __construct(IriConverterInterface $iriConverter)
    {
        $this->iriConverter = $iriConverter;
    }

    public function denormalize($data, $class, $format = null, array $context = [])
    {
        $data['relatedDummy'] = $this->iriConverter->getIriFromResource(resource: RelatedDummy::class, context: ['uri_variables' => ['id' => $data['relatedDummy']]]);

        return $this->denormalizer->denormalize($data, $class, $format, $context + [__CLASS__ => true]);
    }

    public function supportsDenormalization($data, $type, $format = null, array $context = []): bool
    {
        return \in_array($format, ['json', 'jsonld'], true) && is_a($type, Dummy::class, true) && !empty($data['relatedDummy']) && !isset($context[__CLASS__]);
    }

    public function getSupportedTypes(?string $format): array
    {
        return [
            'object' => null,
            '*' => false,
            Dummy::class => true
        ];
    }
}

# Property Normalization Context for Symfony

If you want to change the (de)normalization context of a property, for instance if you want to change the format of the date time, you can do so by using the #[Context] attribute from the Symfony Serializer component.

For instance:

<?php
// api/src/Entity/Book.php

namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Serializer\Annotation\Context;
use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;

#[ORM\Entity]
#[ApiResource]
class Book
{
    #[ORM\Column]
    #[Context([DateTimeNormalizer::FORMAT_KEY => 'Y-m-d'])]
    public ?\DateTimeInterface $publicationDate = null;
}

In the above example, you will receive the book’s data like this:

{
  "@context": "/contexts/Book",
  "@id": "/books/3",
  "@type": "https://schema.org/Book",
  "publicationDate": "1989-06-16"
}

It’s also possible to only change the denormalization or normalization context:

<?php
// api/src/Entity/Book.php

namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Serializer\Annotation\Context;
use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;

#[ORM\Entity]
#[ApiResource]
class Book
{
    #[ORM\Column]
    #[Context(normalizationContext: [DateTimeNormalizer::FORMAT_KEY => 'Y-m-d'])]
    public ?\DateTimeInterface $publicationDate = null;
}

Groups are also supported:

<?php
// api/src/Entity/Book.php

namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Serializer\Annotation\Context;
use Symfony\Component\Serializer\Annotation\Groups;
use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;

#[ORM\Entity]
#[ApiResource]
class Book
{
    #[ORM\Column]
    #[Groups(["extended"])]
    #[Context([DateTimeNormalizer::FORMAT_KEY => \DateTime::RFC3339])]
    #[Context(
        context: [DateTimeNormalizer::FORMAT_KEY => \DateTime::RFC3339_EXTENDED],
        groups: ['extended'],
    )]
    public ?\DateTimeInterface $publicationDate = null;
}

# Calculated Field using Doctrine

Sometimes you need to expose calculated fields. This can be done by leveraging the groups. This time not on a property, but on a method.

<?php
// api/src/Entity/Greeting.php

namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Metadata\GetCollection;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource]
#[GetCollection(normalizationContext: ['groups' => 'greeting:collection:get'])]
class Greeting
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    #[Groups("greeting:collection:get")]
    private ?int $id = null;

    private $a = 1;

    private $b = 2;

    #[ORM\Column]
    #[Groups("greeting:collection:get")]
    public string $name = '';

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

    #[Groups('greeting:collection:get')] // <- MAGIC IS HERE, you can set a group on a method.
    public function getSum(): int
    {
        return $this->a + $this->b;
    }
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Greeting.yaml
App\Entity\Greeting:
    operations:
        ApiPlatform\Metadata\GetCollection:
            normalizationContext:
                groups: 'greeting:collection:get'

# The YAML syntax is only supported for Symfony
# api/config/serializer/Greeting.yaml
App\Entity\Greeting:
    attributes:
        id:
            groups: 'greeting:collection:get'
        name:
            groups: 'greeting:collection:get'
        sum:
            groups: 'greeting:collection:get'

# Changing the Serialization Context Dynamically

Watch the Context Builder & Service Decoration screencast

Let’s imagine a resource where most fields can be managed by any user, but some can be managed only by admin users:

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use Symfony\Component\Serializer\Annotation\Groups;

#[ApiResource(
    normalizationContext: ['groups' => ['book:output']],
    denormalizationContext: ['groups' => ['book:input']],
)]
class Book
{
    // ...

    /**
     * This field can be managed only by an admin
     */
    #[Groups(['book:output', 'admin:input'])]
    public bool $active = false;

    /**
     * This field can be managed by any user
     */
    #[Groups(['book:output', 'book:input'])]
    public string $name;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Book.yaml
App\ApiResource\Book:
    normalizationContext:
        groups: ['book:output']
    denormalizationContext:
        groups: ['book:input']

# The YAML syntax is only supported for Symfony
# api/config/serializer/Book.yaml
App\ApiResource\Book:
    attributes:
        active:
            groups: ['book:output', 'admin:input']
        name:
            groups: ['book:output', 'book:input']

All entry points are the same for all users, so we should find a way to detect if the authenticated user is an admin, and if so dynamically add the admin:input value to deserialization groups in the $context array.

API Platform implements a ContextBuilder, which prepares the context for serialization & deserialization. Let’s decorate this service to override the createFromRequest method:

# The YAML syntax is only supported for Symfony
# api/config/services.yaml
services:
  # ...
  'App\Serializer\BookContextBuilder':
    decorates: 'api_platform.serializer.context_builder'
    arguments: ['@App\Serializer\BookContextBuilder.inner']
    autoconfigure: false

<?php
// api/src/Serializer/BookContextBuilder.php
namespace App\Serializer;

use ApiPlatform\Serializer\SerializerContextBuilderInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Security\Core\Authorization\AuthorizationCheckerInterface;
use App\ApiResource\Book;

final class BookContextBuilder implements SerializerContextBuilderInterface
{
    private $decorated;
    private $authorizationChecker;

    public function __construct(SerializerContextBuilderInterface $decorated, AuthorizationCheckerInterface $authorizationChecker)
    {
        $this->decorated = $decorated;
        $this->authorizationChecker = $authorizationChecker;
    }

    public function createFromRequest(Request $request, bool $normalization, ?array $extractedAttributes = null): array
    {
        $context = $this->decorated->createFromRequest($request, $normalization, $extractedAttributes);
        $resourceClass = $context['resource_class'] ?? null;

        if ($resourceClass === Book::class && isset($context['groups']) && $this->authorizationChecker->isGranted('ROLE_ADMIN') && false === $normalization) {
            $context['groups'][] = 'admin:input';
        }

        return $context;
    }
}

If the user has the ROLE_ADMIN permission and the subject is an instance of Book, admin:input group will be dynamically added to the denormalization context. The $normalization variable lets you check whether the context is for normalization (if TRUE) or denormalization (FALSE).

# Changing the Serialization Context on a Per-item Basis for Symfony

The example above demonstrates how you can modify the normalization/denormalization context based on the current user permissions for all books. Sometimes, however, the permissions vary depending on what book is being processed.

Think of ACL’s: User “A” may retrieve Book “A” but not Book “B”. In this case, we need to leverage the power of the Symfony Serializer and register our own normalizer that adds the group on every single item (note: priority 64 is an example; it is always important to make sure your normalizer gets loaded first, so set the priority to whatever value is appropriate for your application; higher values are loaded earlier):

# api/config/services.yaml
services:
  'App\Serializer\BookAttributeNormalizer':
    arguments: ['@security.token_storage']
    tags:
      - { name: 'serializer.normalizer', priority: 64 }

The Normalizer class is a bit harder to understand, because it must ensure that it is only called once and that there is no recursion. To accomplish this, it needs to be aware of the parent Normalizer instance itself.

Here is an example:

<?php
// api/src/Serializer/BookAttributeNormalizer.php
namespace App\Serializer;

use Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorageInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerAwareInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerAwareTrait;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

class BookAttributeNormalizer implements NormalizerInterface, NormalizerAwareInterface
{
    use NormalizerAwareTrait;

    private const ALREADY_CALLED = 'BOOK_ATTRIBUTE_NORMALIZER_ALREADY_CALLED';

    private $tokenStorage;

    public function __construct(TokenStorageInterface $tokenStorage)
    {
        $this->tokenStorage = $tokenStorage;
    }

    public function normalize($object, $format = null, array $context = [])
    {
        if ($this->userHasPermissionsForBook($object)) {
            $context['groups'][] = 'can_retrieve_book';
        }

        $context[self::ALREADY_CALLED] = true;

        return $this->normalizer->normalize($object, $format, $context);
    }

    public function supportsNormalization($data, $format = null, array $context = [])
    {
        // Make sure we're not called twice
        if (isset($context[self::ALREADY_CALLED])) {
            return false;
        }

        return $data instanceof Book;
    }

    private function userHasPermissionsForBook($object): bool
    {
        // Get permissions from user in $this->tokenStorage
        // for the current $object (book) and
        // return true or false
    }
}

This will add the serialization group can_retrieve_book only if the currently logged-in user has access to the given book instance.

Note: In this example, we use the TokenStorageInterface to verify access to the book instance. However, Symfony provides many useful other services that might be better suited to your use case. For example, the AuthorizationChecker.

# Name Conversion for Symfony

The Serializer Component provides a handy way to map PHP field names to serialized names. See the related Symfony documentation.

To use this feature, declare a new name converter service. For example, you can convert CamelCase to snake_case with the following configuration:

# api/config/services.yaml
services:
  'Symfony\Component\Serializer\NameConverter\CamelCaseToSnakeCaseNameConverter': ~

# api/config/packages/api_platform.yaml
api_platform:
  name_converter: 'Symfony\Component\Serializer\NameConverter\CamelCaseToSnakeCaseNameConverter'

If symfony’s MetadataAwareNameConverter is available it’ll be used by default. If you specify one in ApiPlatform configuration, it’ll be used. Note that you can use decoration to benefit from this name converter in your own implementation.

# Decorating a Serializer and Adding Extra Data for Symfony

In the following example, we will see how we add extra information to the serialized output. Here is how we add the date on each request in GET:

# api/config/services.yaml
services:
  'App\Serializer\ApiNormalizer':
    # By default .inner is passed as argument
    decorates: 'api_platform.jsonld.normalizer.item'

Note: this normalizer will work only for JSON-LD format, if you want to process JSON data too, you have to decorate another service:

# Need a different name to avoid duplicate YAML key
'app.serializer.normalizer.item.json':
  class: 'App\Serializer\ApiNormalizer'
  decorates: 'api_platform.serializer.normalizer.item'

<?php
// api/src/Serializer/ApiNormalizer.php

namespace App\Serializer;

use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
use Symfony\Component\Serializer\SerializerAwareInterface;
use Symfony\Component\Serializer\SerializerInterface;

final class ApiNormalizer implements NormalizerInterface, DenormalizerInterface, SerializerAwareInterface
{
    private $decorated;

    public function __construct(NormalizerInterface $decorated)
    {
        if (!$decorated instanceof DenormalizerInterface) {
            throw new \InvalidArgumentException(sprintf('The decorated normalizer must implement the %s.', DenormalizerInterface::class));
        }

        $this->decorated = $decorated;
    }

    public function supportsNormalization($data, $format = null)
    {
        return $this->decorated->supportsNormalization($data, $format);
    }

    public function normalize($object, $format = null, array $context = [])
    {
        $data = $this->decorated->normalize($object, $format, $context);
        if (is_array($data)) {
            $data['date'] = date(\DateTime::RFC3339);
        }

        return $data;
    }

    public function supportsDenormalization($data, $type, $format = null)
    {
        return $this->decorated->supportsDenormalization($data, $type, $format);
    }

    public function denormalize($data, string $type, string $format = null, array $context = [])
    {
        return $this->decorated->denormalize($data, $type, $format, $context);
    }

    public function setSerializer(SerializerInterface $serializer)
    {
        if($this->decorated instanceof SerializerAwareInterface) {
            $this->decorated->setSerializer($serializer);
        }
    }
}

# Entity/Model Identifier Case

API Platform is able to guess the entity/model identifier using Doctrine metadata (ORM, MongoDB ODM) or Laravel Eloquent metadata (ORM).

For ORM, it also supports Doctrine composite identifiers and Eloquent composite identifiers.

If you are not using the Doctrine ORM or MongoDB ODM Provider, you must explicitly mark the identifier using the identifier attribute of the ApiPlatform\Metadata\ApiProperty annotation. For example:

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Metadata\ApiProperty;

#[ApiResource]
class Book
{
    // ...

    #[ApiProperty(identifier: true)]
    private $id;

    /**
     * This field can be managed only by an admin
     */
    public bool $active = false;

    /**
     * This field can be managed by any user
     */
    public string $name;

    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/properties.yaml
properties:
  App\ApiResource\Book:
    id:
      identifier: true

<?xml version="1.0" encoding="UTF-8" ?>
<!-- The XML syntax is only supported for Symfony -->
<!-- api/config/api_platform/properties.xml -->

<properties xmlns="https://api-platform.com/schema/metadata/properties-3.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="https://api-platform.com/schema/metadata/properties-3.0
           https://api-platform.com/schema/metadata/properties-3.0.xsd">
    <property resource="App\ApiResource\Product" name="id" identifier="true" />
</properties>

In some cases, you will want to set the identifier of a resource from the client (e.g. a client-side generated UUID, or a slug). In such cases, you must make the identifier property a writable class property. Specifically, to use client-generated IDs, you must do the following:

create a setter for the identifier of the entity/model (e.g. public function setId(string $id)) or make it a public property ,
add the denormalization group to the property (only if you use a specific denormalization group), and,
if you use Doctrine ORM, be sure to not mark this property with the @GeneratedValue annotation or use the NONE value

# Embedding the JSON-LD Context

By default, the generated JSON-LD context (@context) is only referenced by an IRI. A client that uses JSON-LD must send a second HTTP request to retrieve it:

{
  "@context": "/contexts/Book",
  "@id": "/books/62",
  "@type": "Book",
  "name": "My awesome book",
  "author": "/people/59"
}

You can configure API Platform to embed the JSON-LD context in the root document by adding the jsonld_embed_context attribute to the #[ApiResource] annotation:

<?php
// api/src/ApiResource/Book.php with Symfony or app/ApiResource/Book.php with Laravel
namespace App\ApiResource;

use ApiPlatform\Metadata\ApiResource;

#[ApiResource(normalizationContext: ['jsonld_embed_context' => true])]
class Book
{
    // ...
}

# The YAML syntax is only supported for Symfony
# api/config/api_platform/resources/Book.yaml
App\ApiResource\Book:
  normalizationContext:
    jsonldEmbedContext: true

The JSON output will now include the embedded context:

{
  "@context": {
    "@vocab": "http://localhost:8000/apidoc#",
    "hydra": "http://www.w3.org/ns/hydra/core#",
    "name": "https://schema.org/name",
    "author": "https://schema.org/author"
  },
  "@id": "/books/62",
  "@type": "Book",
  "name": "My awesome book",
  "author": "/people/59"
}

# Collection Relation using Doctrine

This is a special case where, in an entity, you have a toMany relation. By default, Doctrine will use an ArrayCollection to store your values. This is fine when you have a read operation, but when you try to write you can observe an issue where the response is not reflecting the changes correctly. It can lead to client errors even though the update was correct. Indeed, after an update on this relation, the collection looks wrong because ArrayCollection’s indices are not sequential. To change this, we recommend to use a getter that returns $collectionRelation->getValues(). Thanks to this, the relation is now a real array which is sequentially indexed.

<?php
// api/src/Entity/Brand.php
namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ApiResource]
final class Brand
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    private ?int $id = null;

    #[ORM\ManyToMany(targetEntity: Car::class, inversedBy: 'brands')]
    #[ORM\JoinTable(name: 'CarToBrand')]
    #[ORM\JoinColumn(name: 'brand_id', referencedColumnName: 'id', nullable: false)]
    #[ORM\InverseJoinColumn(name: 'car_id', referencedColumnName: 'id', nullable: false)]
    private $cars;

    public function __construct()
    {
        $this->cars = new ArrayCollection();
    }

    public function addCar(DummyCar $car)
    {
        $this->cars[] = $car;
    }

    public function removeCar(DummyCar $car)
    {
        $this->cars->removeElement($car);
    }

    public function getCars()
    {
        return $this->cars->getValues();
    }

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

For reference please check #1534.