Adding MorphOne field.

docs/docs/4.0/filtering/filtering.md

@@ -336,6 +336,26 @@ public function foo() {
 }
 ```
 
+## Related eager
+
+You can define an eager field to represent and serialize your data:
+
+```php
+
+use Binaryk\LaravelRestify\Fields\BelongsTo;
+use Binaryk\LaravelRestify\Fields\MorphToMany;
+
+public static function related(): array
+{
+    return [
+        'user' => BelongsTo::make('user', 'user', UserRepository::class),
+        'comments' => MorphToMany::make('comments', 'comments', CommentRepository::class),
+    ];
+}
+```
+
+You have the following relations available: `MorphToMany`, `MorphOne` `BelongsTo`, `HasOne`, `HasMany`, `BelongsToMany`.
+
 ### Custom data format
 
 You can use a custom related cast class (aka transformer). You can do so by modifying the `restify.casts.related`

src/Fields/MorphOne.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace Binaryk\LaravelRestify\Fields;
+
+class MorphOne extends BelongsTo
+{
+}

tests/Fields/MorphOneFieldTest.php

@@ -0,0 +1,81 @@
+<?php
+
+namespace Binaryk\LaravelRestify\Tests\Fields;
+
+use Binaryk\LaravelRestify\Fields\BelongsTo;
+use Binaryk\LaravelRestify\Fields\MorphOne;
+use Binaryk\LaravelRestify\Http\Requests\RestifyRequest;
+use Binaryk\LaravelRestify\Repositories\Repository;
+use Binaryk\LaravelRestify\Restify;
+use Binaryk\LaravelRestify\Tests\Fixtures\Post\Post;
+use Binaryk\LaravelRestify\Tests\Fixtures\User\User;
+use Binaryk\LaravelRestify\Tests\Fixtures\User\UserRepository;
+use Binaryk\LaravelRestify\Tests\IntegrationTest;
+
+class MorphOneFieldTest extends IntegrationTest
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->authenticate();
+
+        Restify::repositories([
+            PostWithMophOneRepository::class,
+        ]);
+    }
+
+    protected function tearDown(): void
+    {
+        parent::tearDown();
+        Repository::clearResolvedInstances();
+    }
+
+    public function test_morph_one_present_on_show_when_specified_related()
+    {
+        $post = factory(Post::class)->create([
+            'user_id' => factory(User::class),
+        ]);
+
+        $relationships = $this->get(PostWithMophOneRepository::uriKey() . "/$post->id?related=user")
+            ->assertJsonStructure([
+                'data' => [
+                    'relationships' => [
+                        'user' => [
+                            'id',
+                            'type',
+                            'attributes',
+                        ],
+                    ],
+                ],
+            ])
+            ->json('data.relationships');
+
+        $this->assertNotNull($relationships);
+
+        $relationships = $this->get(PostWithMophOneRepository::uriKey() . "/$post->id")
+            ->json('data.relationships');
+
+        $this->assertNull($relationships);
+    }
+}
+
+class PostWithMophOneRepository extends Repository
+{
+    public static $model = Post::class;
+
+    public static function getRelated()
+    {
+        return [
+            'user' => BelongsTo::make('user', 'user', UserRepository::class),
+        ];
+    }
+
+    public function fields(RestifyRequest $request)
+    {
+        return [
+            field('title'),
+
+            MorphOne::make('user', 'user', UserRepository::class),
+        ];
+    }
+}