fix(jsonapi): unwrap relationship linkage before denormalization

src/JsonApi/Serializer/ItemNormalizerTrait.php

@@ -79,10 +79,10 @@ public function denormalize(mixed $data, string $type, ?string $format = null, a
             }
         }
 
-        $dataToDenormalize = array_merge(
-            $data['data']['attributes'] ?? [],
-            $data['data']['relationships'] ?? []
-        );
+        $dataToDenormalize = $data['data']['attributes'] ?? [];
+        foreach ($data['data']['relationships'] ?? [] as $name => $relationship) {
+            $dataToDenormalize[$name] = \is_array($relationship) && \array_key_exists('data', $relationship) ? $relationship['data'] : $relationship;
+        }
 
         // Surface the client-generated id so the entity setter receives it.
         if ($isPostOperation && $allowClientGeneratedId && isset($data['data']['id'])) {

tests/Fixtures/TestBundle/ApiResource/Issue8702/Issue8702Author.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\Issue8702;
+
+use ApiPlatform\Metadata\ApiProperty;
+use ApiPlatform\Metadata\ApiResource;
+use ApiPlatform\Metadata\Get;
+
+#[ApiResource(
+    shortName: 'Issue8702Author',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    operations: [new Get(uriTemplate: '/issue8702_authors/{id}', provider: [self::class, 'provide'])],
+)]
+final class Issue8702Author
+{
+    public function __construct(
+        #[ApiProperty(identifier: true)]
+        public int $id = 1,
+        public string $name = 'Frank Herbert',
+    ) {
+    }
+
+    public static function provide(): self
+    {
+        return new self();
+    }
+}

tests/Fixtures/TestBundle/ApiResource/Issue8702/Issue8702Book.php

@@ -0,0 +1,53 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\Issue8702;
+
+use ApiPlatform\Metadata\ApiProperty;
+use ApiPlatform\Metadata\ApiResource;
+use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\Post;
+
+#[ApiResource(
+    shortName: 'Issue8702Book',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    operations: [
+        new Get(uriTemplate: '/issue8702_books/{id}', provider: [self::class, 'provide']),
+        new Post(uriTemplate: '/issue8702_books', processor: [self::class, 'process']),
+    ],
+)]
+final class Issue8702Book
+{
+    /**
+     * @param list<Issue8702Author> $coAuthors
+     */
+    public function __construct(
+        #[ApiProperty(identifier: true)]
+        public int $id = 1,
+        public string $title = 'Dune',
+        public ?Issue8702Author $author = null,
+        public array $coAuthors = [],
+        public array $payload = [],
+    ) {
+    }
+
+    public static function provide(): null
+    {
+        return null;
+    }
+
+    public static function process(self $data): self
+    {
+        return $data;
+    }
+}

tests/Functional/JsonApi/ConstructorRelationshipLinkageTest.php

@@ -0,0 +1,75 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Functional\JsonApi;
+
+use ApiPlatform\Test\ApiTestCase;
+use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\Issue8702\Issue8702Author;
+use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\Issue8702\Issue8702Book;
+use ApiPlatform\Tests\SetupClassResourcesTrait;
+use Symfony\Contracts\HttpClient\ResponseInterface;
+
+final class ConstructorRelationshipLinkageTest extends ApiTestCase
+{
+    use SetupClassResourcesTrait;
+
+    protected static ?bool $alwaysBootKernel = false;
+
+    /**
+     * @return class-string[]
+     */
+    public static function getResources(): array
+    {
+        return [Issue8702Author::class, Issue8702Book::class];
+    }
+
+    public function testToOneResourceLinkage(): void
+    {
+        $response = $this->post(['author' => ['data' => ['type' => 'Issue8702Author', 'id' => '/issue8702_authors/1']]]);
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertSame('/issue8702_authors/1', $response->toArray()['data']['relationships']['author']['data']['id']);
+    }
+
+    public function testToOneNullLinkage(): void
+    {
+        $response = $this->post(['author' => ['data' => null]]);
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertNull($response->toArray()['data']['relationships']['author']['data']);
+    }
+
+    public function testToManyResourceLinkage(): void
+    {
+        $response = $this->post(['coAuthors' => ['data' => [['type' => 'Issue8702Author', 'id' => '/issue8702_authors/1']]]]);
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertSame('/issue8702_authors/1', $response->toArray()['data']['relationships']['coAuthors']['data'][0]['id']);
+    }
+
+    public function testAttributeWithDataKeyIsNotUnwrapped(): void
+    {
+        $response = $this->post([], ['payload' => ['data' => [1, 2], 'x' => 3]]);
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertSame(['data' => [1, 2], 'x' => 3], $response->toArray()['data']['attributes']['payload']);
+    }
+
+    private function post(array $relationships, array $attributes = []): ResponseInterface
+    {
+        return self::createClient()->request('POST', '/issue8702_books', [
+            'headers' => ['Accept' => 'application/vnd.api+json', 'Content-Type' => 'application/vnd.api+json'],
+            'json' => ['data' => ['type' => 'Issue8702Book', 'attributes' => ['title' => 'A'] + $attributes, 'relationships' => $relationships]],
+        ]);
+    }
+}