fix: assign missing value when a relation resource is null

src/Resources/Relationship.php

@@ -145,7 +145,8 @@ public function toArray(mixed $request, bool $included = true): array
     {
         $value = $this->whenIncluded && !$included
             ? new MissingValue
-            : value($this->value);
+            : value($this->value)
+            ?? new MissingValue;
 
         if ($this->asCollection && !is_subclass_of($this->resource, ResourceCollection::class)) {
             $resource = $this->resource::collection($value);

src/Support/Values.php

@@ -29,6 +29,7 @@ public static function mergeValues(iterable $data): array
             $index++;
 
             if (is_iterable($value)) {
+                /** @var array<array-key, mixed> $value */
                 $data[$key] = self::mergeValues($value);
 
                 continue;

tests/Feature/SchemaTest.php

@@ -17,8 +17,10 @@ public function testSchema()
         $post = new Skeleton(PostResource::class, 'post', ['state', 'title', 'content']);
         $comment = new Skeleton(CommentResource::class, 'comment', ['content']);
 
+        $user->relationships['main-post'] = $post;
         $user->relationships['posts'] = $post;
         $user->relationships['comments'] = $comment;
+        $user->loads['main-post'] = 'post';
         $user->loads['posts'] = 'posts';
         $user->loads['comments'] = [
             'comments' => fn(Builder $q) => $q->where('content', 'like', '%e%')

tests/Feature/User/ResourceTest.php

@@ -166,6 +166,7 @@ private function getJsonResult(User $user, ?array $attributes = null, ?array $re
                     'email' => $user->email,
                 ], array_fill_keys($attributes ?? ['name', 'email'], true))),
                 'relationships' => [
+                    'main-post' => [],
                     'posts' => array_filter([
                         'data' => $withIncluded ? $user->posts->map(fn(Post $post) => ['type' => 'post', 'id' => $post->id])->all() : null,
                         'links' => [

tests/Unit/Resources/RelationshipTest.php

@@ -144,6 +144,23 @@ public function testToArrayMissingValue()
         ], $relation->toArray(new Request(), false));
     }
 
+    public function testToArrayNullValue()
+    {
+        $resource = $this->getJsonResourceNullValue();
+        $relation = (new Relationship($resource::class, fn() => $resource->resource))->withLinks(fn() => [
+            'self' => 'link'
+        ])->withMeta(fn() => [
+            'hash' => 'azerty'
+        ]);
+
+        $this->assertEquals([
+            'data' => [
+                'links' => ['self' => 'link'],
+                'meta' => ['hash' => 'azerty'],
+            ],
+        ], $relation->toArray(new Request(), false));
+    }
+
     public function testCustomValue()
     {
         $relation = (new Relationship(Collection::class, fn() => ['foo' => 'bar']))
@@ -217,6 +234,22 @@ public function toArray($request, bool $minimal = false): array
         };
     }
 
+    private function getJsonResourceNullValue()
+    {
+        return new class(null) extends JsonResource implements Resourceable {
+            public function toArray($request, bool $minimal = false): array
+            {
+                return [
+                    'type' => 'my-model',
+                    'id' => $this->id,
+                    'attributes' => [
+                        'foo' => 'bar'
+                    ]
+                ];
+            }
+        };
+    }
+
     private function getResourceCollection()
     {
         return new class(collect()) extends ResourceCollection implements Resourceable {

tests/app/Http/Resources/UserResource.php

@@ -39,6 +39,8 @@ protected function toResourceMeta(Request $request): ?iterable
     protected function toRelationships(Request $request): iterable
     {
         return [
+            'main-post' => $this->one(PostResource::class, fn() => $this->resource->post)
+                ->withLoad('post'),
             'posts' => PostResource::relationship(fn() => $this->resource->posts, fn() => [
                 'self' => "https://api.example.com/user/{$this->resource->id}/relationships/posts",
                 'related' => "https://api.example.com/user/{$this->resource->id}/posts",

tests/app/Models/User.php

@@ -4,6 +4,7 @@
 
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Test\app\Factories\UserFactory;
 
@@ -14,6 +15,7 @@
  * @property \DateTimeInterface $created_at
  * @property \DateTimeInterface $updated_at
  *
+ * @property-read Post|null $post
  * @property-read Post[]|\Illuminate\Support\Collection<Post> $posts
  * @property-read Comment[]|\Illuminate\Support\Collection<Comment> $comments
  */
@@ -26,6 +28,11 @@ protected static function newFactory(): UserFactory
         return new UserFactory();
     }
 
+    public function post(): BelongsTo
+    {
+        return $this->belongsTo(Post::class);
+    }
+
     public function posts(): HasMany
     {
         return $this->hasMany(Post::class);

tests/app/migrations.php

@@ -36,11 +36,26 @@ public function tables()
         ];
     }
 
+    public function changes()
+    {
+        return [
+            'users' => function (Blueprint $table) {
+                $table->foreignId('post_id')
+                    ->nullable()
+                    ->after('password')
+                    ->constrained('posts');
+            },
+        ];
+    }
+
     public function up()
     {
         foreach ($this->tables() as $table => $blueprint) {
             Schema::create($table, $blueprint);
         }
+        foreach ($this->changes() as $table => $blueprint) {
+            Schema::table($table, $blueprint);
+        }
     }
 
     public function down()