diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
new file mode 100644
index 0000000..66a983e
--- /dev/null
+++ b/.github/FUNDING.yml
@@ -0,0 +1,2 @@
+custom: "https://www.paypal.me/f3ath"
+
diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
new file mode 100644
index 0000000..eae93ab
--- /dev/null
+++ b/.github/workflows/php.yml
@@ -0,0 +1,36 @@
+name: PHP Composer
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Validate composer.json and composer.lock
+      run: composer validate --strict
+
+    - name: Cache Composer packages
+      id: composer-cache
+      uses: actions/cache@v3
+      with:
+        path: vendor
+        key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-php-
+
+    - name: Install dependencies
+      run: composer install --prefer-dist --no-progress
+
+    - name: Run test suite
+      run: composer run-script test
diff --git a/.gitignore b/.gitignore
index f22a32d..c195581 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,5 @@ composer.phar
 /vendor/
 /composer.lock
 /.php_cs.cache
+.php-cs-fixer.cache
+.phpunit.result.cache
diff --git a/.php-cs-fixer.php b/.php-cs-fixer.php
new file mode 100644
index 0000000..acd1414
--- /dev/null
+++ b/.php-cs-fixer.php
@@ -0,0 +1,16 @@
+<?php
+
+$finder = PhpCsFixer\Finder::create()
+    ->exclude('somedir')
+    ->notPath('src/Symfony/Component/Translation/Tests/fixtures/resources.php')
+    ->in(__DIR__);
+
+$config = new PhpCsFixer\Config();
+return $config->setRules([
+    '@PSR12' => true,
+    'array_syntax' => ['syntax' => 'short'],
+    'braces' => [
+        'allow_single_line_closure' => true,
+        'position_after_functions_and_oop_constructs' => 'same'],
+])
+    ->setFinder($finder);
\ No newline at end of file
diff --git a/.php_cs.dist b/.php_cs.dist
deleted file mode 100644
index 06b82a3..0000000
--- a/.php_cs.dist
+++ /dev/null
@@ -1,53 +0,0 @@
-<?php
-$finder = PhpCsFixer\Finder::create()
-    ->files()
-    ->name('*.php')
-    ->in(__DIR__ . '/examples')
-    ->in(__DIR__ . '/src')
-    ->in(__DIR__ . '/test');
-return PhpCsFixer\Config::create()
-    ->setUsingCache(true)
-    ->setRiskyAllowed(true)
-    ->setRules([
-        '@PSR2' => true,
-        'array_syntax' => ['syntax' => 'short'],
-        'binary_operator_spaces' => true,
-        'cast_spaces' => true,
-        'concat_space' => true,
-        'declare_strict_types' => true,
-        'include' => true,
-        'is_null' => true,
-        'lowercase_cast' => true,
-        'mb_str_functions' => true,
-        'method_separation' => true,
-        'native_function_casing' => true,
-        'no_blank_lines_after_class_opening' => true,
-        'no_blank_lines_after_phpdoc' => true,
-        'no_empty_statement' => true,
-        'no_extra_consecutive_blank_lines' => true,
-        'no_leading_import_slash' => true,
-        'no_leading_namespace_whitespace' => true,
-        'no_trailing_comma_in_singleline_array' => true,
-        'no_unused_imports' => true,
-        'no_whitespace_in_blank_line' => true,
-        'object_operator_without_whitespace' => true,
-        'ordered_imports' => true,
-        'phpdoc_align' => true,
-        'phpdoc_indent' => true,
-        'phpdoc_no_access' => true,
-        'phpdoc_no_package' => true,
-        'phpdoc_order' => true,
-        'phpdoc_scalar' => true,
-        'phpdoc_trim' => true,
-        'phpdoc_types' => true,
-        'psr0' => true,
-        'short_scalar_cast' => true,
-        'single_blank_line_before_namespace' => true,
-        'single_quote' => true,
-        'standardize_not_equals' => true,
-        'strict_comparison' => true,
-        'ternary_operator_spaces' => true,
-        'trailing_comma_in_multiline_array' => true,
-        'whitespace_after_comma_in_array' => true,
-    ])
-    ->setFinder($finder);
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index bef7869..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,11 +0,0 @@
-language: php
-php:
-  - '7.2'
-
-before_script:
-  - composer install
-  - mkdir build/logs -p
-
-script:
-  - vendor/bin/php-cs-fixer fix -v --dry-run
-  - phpunit --coverage-clover build/logs/clover.xml
diff --git a/CHANGELOG.md b/CHANGELOG.md
index bebaa62..22fd9fe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,42 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
-## [2.0.0] - 2018-02-26
-V2 initial release
+## [3.0.0] - 2022-07-26
+### Changed
+- The package is migrated to PHP 8.1
 
-[Unreleased]: https://github.com/json-api-php/json-api/compare/2.0.0...HEAD
+### Removed
+- Support for PHP 7 and older versions
+- Compound document validation logic is dropped
+
+## [2.2.0] - 2020-10-12
+### Added
+- `NewResourceObject` to allow omitting `id` in resources to-be-created (#108)
+
+## [2.1.2] - 2020-03-16
+### Fixed
+- Related links must be allowed inside relationship documents (#104)
+
+## [2.1.1] - 2019-12-19
+### Fixed
+- `ResourceIdentifier` does not allow multiple meta members (#99)
+
+## [2.1.0] - 2019-02-25
+### Fixed
+- Relationship without data property (#92)
+
+## [2.0.1] - 2018-12-31
+### Changed
+- Downgraded min required php version to 7.1
+
+## 2.0.0 - 2018-02-26
+### Added
+- v2 initial release
+
+[Unreleased]: https://github.com/json-api-php/json-api/compare/3.0.0...HEAD
+[3.0.0]: https://github.com/json-api-php/json-api/compare/2.2.2...3.0.0
+[2.2.0]: https://github.com/json-api-php/json-api/compare/2.1.2...2.2.0
+[2.1.2]: https://github.com/json-api-php/json-api/compare/2.1.1...2.1.2
+[2.1.1]: https://github.com/json-api-php/json-api/compare/2.1.0...2.1.1
+[2.1.0]: https://github.com/json-api-php/json-api/compare/2.0.1...2.1.0
+[2.0.1]: https://github.com/json-api-php/json-api/compare/2.0.0...2.0.1
diff --git a/README.md b/README.md
index 1677ff1..2272090 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,5 @@
 # [JSON API](http://jsonapi.org) spec implemented in PHP 7. Immutable
 
-**This is v2 of the implementation. For v1 click [here](https://github.com/json-api-php/json-api/tree/v1).**
-
 The goal of this library is to ensure strict validity of JSON API documents being produced.
 
 JSON:
@@ -65,13 +63,14 @@ First, take a look at the examples. All of them are runnable.
 - [Simple Document](./examples/simple_doc.php) (the same as above)
 - [Extensive Compound Document](./examples/compound_doc.php)
 
-The library API and use-cases are expressed in comprehensive suite of tests.
+The library API and use-cases are expressed in a comprehensive suite of tests.
 - Data Documents (containing primary data)
     -  [with a single Resource Object](./test/DataDocument/SingleResourceObjectTest.php)
     -  [with a single Resource Identifier](./test/DataDocument/SingleResourceIdentifierTest.php)
     -  [with null data](./test/DataDocument/NullDataTest.php)
     -  [with multiple Resource Objects](./test/DataDocument/ManyResourceObjectsTest.php)
     -  [with multiple Resource Identifiers](./test/DataDocument/ManyResourceIdentifiersTest.php)
+    -  [with a new Resource (no id)](./test/NewResourceObjectTest.php)
 - [Compound Documents](./test/CompoundDocumentTest.php)
 - [Error Documents](./test/ErrorDocumentTest.php)
 - [Meta Documents (containing neither data nor errors)](./test/MetaDocumentTest.php)
diff --git a/composer.json b/composer.json
index 1cdeb90..5a7e2b9 100644
--- a/composer.json
+++ b/composer.json
@@ -1,6 +1,6 @@
 {
   "name": "json-api-php/json-api",
-  "description": "An attempt to express JSON API specs (jsonapi.org) in object-oriented way as a set of PHP 7 classes",
+  "description": "JSON API specs (jsonapi.org) as a set of PHP classes",
   "type": "library",
   "prefer-stable": true,
   "license": "MIT",
@@ -11,11 +11,12 @@
     }
   ],
   "require": {
-    "php": ">=7.2"
+    "php": ">=8.1",
+    "ext-json": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7.0",
-    "friendsofphp/php-cs-fixer": "^2.2"
+    "phpunit/phpunit": "^9.0",
+    "friendsofphp/php-cs-fixer": "^3.9"
   },
   "autoload": {
     "psr-4": {
diff --git a/examples/compound_doc.php b/examples/compound_doc.php
index ddb6fd9..1d0cbbb 100644
--- a/examples/compound_doc.php
+++ b/examples/compound_doc.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 use JsonApiPhp\JsonApi\Attribute;
 use JsonApiPhp\JsonApi\CompoundDocument;
@@ -33,8 +35,8 @@
     new Attribute('body', 'First!'),
     new SelfLink('http://example.com/comments/5'),
     new ToOne('author', new ResourceIdentifier('people', '2'))
-
 );
+
 $comment12 = new ResourceObject(
     'comments',
     '12',
diff --git a/examples/simple_doc.php b/examples/simple_doc.php
index a74263a..eb7c5f4 100644
--- a/examples/simple_doc.php
+++ b/examples/simple_doc.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 require_once __DIR__.'/../vendor/autoload.php';
 
 use JsonApiPhp\JsonApi\Attribute;
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 48bfa68..21f9c66 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,18 +1,13 @@
-<?xml version="1.0" encoding="UTF-8" ?>
-<phpunit
-        bootstrap="vendor/autoload.php"
-        stopOnFailure="false"
-        verbose="true"
-        colors="true"
-        syntaxCheck="true">
-    <testsuites>
-        <testsuite name="Main">
-            <directory>./test</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist>
-            <directory suffix=".php">./src/</directory>
-        </whitelist>
-    </filter>
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" bootstrap="vendor/autoload.php" stopOnFailure="false" verbose="true" colors="true" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
+  <coverage>
+    <include>
+      <directory suffix=".php">./src/</directory>
+    </include>
+  </coverage>
+  <testsuites>
+    <testsuite name="Main">
+      <directory>./test</directory>
+    </testsuite>
+  </testsuites>
 </phpunit>
diff --git a/src/Attribute.php b/src/Attribute.php
index 69c6621..8276746 100644
--- a/src/Attribute.php
+++ b/src/Attribute.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
@@ -8,20 +10,22 @@
 /**
  * @see http://jsonapi.org/format/#document-resource-object-attributes
  */
-final class Attribute implements ResourceField
-{
+final class Attribute implements ResourceField {
     use ResourceFieldTrait;
-    private $val;
 
-    public function __construct(string $name, $val)
-    {
+    private string|int|float|bool|null|array|object $val;
+
+    public function __construct(string $name, $val) {
         $this->validateFieldName($name);
         $this->name = $name;
         $this->val = $val;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'attributes')->{$this->name} = $this->val;
     }
 }
diff --git a/src/CompoundDocument.php b/src/CompoundDocument.php
index 7e84887..7a15492 100644
--- a/src/CompoundDocument.php
+++ b/src/CompoundDocument.php
@@ -1,26 +1,25 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
+use JsonSerializable;
 
 /**
  * A Document with the "included" member
  * @see http://jsonapi.org/format/#document-compound-documents
  */
-final class CompoundDocument implements \JsonSerializable
-{
-    private $doc;
+final class CompoundDocument implements JsonSerializable {
+    private object $doc;
 
-    public function __construct(PrimaryData $data, Included $included, DataDocumentMember ...$members)
-    {
-        $included->validateLinkage($data);
+    public function __construct(PrimaryData $data, Included $included, DataDocumentMember ...$members) {
         $this->doc = combine($data, $included, ...$members);
     }
 
-    public function jsonSerialize()
-    {
+    public function jsonSerialize(): object {
         return $this->doc;
     }
 }
diff --git a/src/DataDocument.php b/src/DataDocument.php
index d8b986d..20d0e7c 100644
--- a/src/DataDocument.php
+++ b/src/DataDocument.php
@@ -1,25 +1,25 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
+use JsonSerializable;
 
 /**
  * A Document containing the "data" member
  * @see http://jsonapi.org/format/#document-top-level
  */
-final class DataDocument implements \JsonSerializable
-{
-    private $value;
+final class DataDocument implements JsonSerializable {
+    private object $value;
 
-    public function __construct(PrimaryData $data, DataDocumentMember ...$members)
-    {
+    public function __construct(PrimaryData $data, DataDocumentMember ...$members) {
         $this->value = combine($data, ...$members);
     }
 
-    public function jsonSerialize()
-    {
+    public function jsonSerialize(): object {
         return $this->value;
     }
 }
diff --git a/src/EmptyRelationship.php b/src/EmptyRelationship.php
new file mode 100644
index 0000000..2c123bb
--- /dev/null
+++ b/src/EmptyRelationship.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi;
+
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
+use JsonApiPhp\JsonApi\Internal\ResourceField;
+use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
+
+/**
+ * A relationship with no data
+ */
+class EmptyRelationship implements ResourceField {
+    use ResourceFieldTrait;
+
+    private object $obj;
+
+    public function __construct(
+        private readonly string $name,
+        RelationshipMember $member,
+        RelationshipMember ...$members
+    ) {
+        $this->obj = combine($member, ...$members);
+    }
+
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
+        child($o, 'relationships')->{$this->name} = $this->obj;
+    }
+}
diff --git a/src/Error.php b/src/Error.php
index 65a012d..8a222fe 100644
--- a/src/Error.php
+++ b/src/Error.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
@@ -9,20 +11,21 @@
  * An Error Object
  * @see
  */
-final class Error implements ErrorDocumentMember
-{
-    private $error;
+final class Error implements ErrorDocumentMember {
+    private readonly object $error;
 
-    public function __construct(ErrorMember ...$members)
-    {
-        $this->error = (object) [];
+    public function __construct(ErrorMember ...$members) {
+        $this->error = (object)[];
         foreach ($members as $member) {
             $member->attachTo($this->error);
         }
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->errors[] = $this->error;
     }
 }
diff --git a/src/Error/Code.php b/src/Error/Code.php
index eafbc72..40455dd 100644
--- a/src/Error/Code.php
+++ b/src/Error/Code.php
@@ -1,26 +1,23 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
-final class Code implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $code;
-
+final class Code implements ErrorMember {
     /**
      * @param string $code an application-specific error code, expressed as a string value
      */
-    public function __construct(string $code)
-    {
-        $this->code = $code;
+    public function __construct(private readonly string $code) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->code = $this->code;
     }
 }
diff --git a/src/Error/Detail.php b/src/Error/Detail.php
index 5950b46..217c43d 100644
--- a/src/Error/Detail.php
+++ b/src/Error/Detail.php
@@ -1,26 +1,23 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
-final class Detail implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $detail;
-
+final class Detail implements ErrorMember {
     /**
      * @param string $detail a human-readable explanation specific to this occurrence of the problem.
      */
-    public function __construct(string $detail)
-    {
-        $this->detail = $detail;
+    public function __construct(private readonly string $detail) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->detail = $this->detail;
     }
 }
diff --git a/src/Error/Id.php b/src/Error/Id.php
index 3553f02..371884f 100644
--- a/src/Error/Id.php
+++ b/src/Error/Id.php
@@ -1,26 +1,23 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
-final class Id implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $id;
-
+final class Id implements ErrorMember {
     /**
      * @param string $id a unique identifier for this particular occurrence of the problem
      */
-    public function __construct(string $id)
-    {
-        $this->id = $id;
+    public function __construct(private readonly string $id) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->id = $this->id;
     }
 }
diff --git a/src/Error/SourceParameter.php b/src/Error/SourceParameter.php
index 613967a..d344d64 100644
--- a/src/Error/SourceParameter.php
+++ b/src/Error/SourceParameter.php
@@ -1,27 +1,25 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
-use function JsonApiPhp\JsonApi\child;
 
-final class SourceParameter implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $parameter;
+use function JsonApiPhp\JsonApi\child;
 
+final class SourceParameter implements ErrorMember {
     /**
      * @param string $parameter a string indicating which URI query parameter caused the error.
      */
-    public function __construct(string $parameter)
-    {
-        $this->parameter = $parameter;
+    public function __construct(private readonly string $parameter) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'source')->parameter = $this->parameter;
     }
 }
diff --git a/src/Error/SourcePointer.php b/src/Error/SourcePointer.php
index bfc6394..c33d0b1 100644
--- a/src/Error/SourcePointer.php
+++ b/src/Error/SourcePointer.php
@@ -1,24 +1,25 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
-use function JsonApiPhp\JsonApi\child;
 
-final class SourcePointer implements ErrorMember
-{
-    private $pointer;
+use function JsonApiPhp\JsonApi\child;
 
+final class SourcePointer implements ErrorMember {
     /**
      * @param string $pointer JSON Pointer [RFC6901] to the associated entity in the request document
      */
-    public function __construct(string $pointer)
-    {
-        $this->pointer = $pointer;
+    public function __construct(private readonly string $pointer) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'source')->pointer = $this->pointer;
     }
 }
diff --git a/src/Error/Status.php b/src/Error/Status.php
index a61031f..2083373 100644
--- a/src/Error/Status.php
+++ b/src/Error/Status.php
@@ -1,26 +1,23 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
-final class Status implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $status;
-
+final class Status implements ErrorMember {
     /**
      * @param string $status the HTTP status code applicable to this problem, expressed as a string value
      */
-    public function __construct(string $status)
-    {
-        $this->status = $status;
+    public function __construct(private readonly string $status) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->status = $this->status;
     }
 }
diff --git a/src/Error/Title.php b/src/Error/Title.php
index d61a706..1352eb5 100644
--- a/src/Error/Title.php
+++ b/src/Error/Title.php
@@ -1,27 +1,24 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Error;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
-final class Title implements ErrorMember
-{
-    /**
-     * @var string
-     */
-    private $title;
-
+final class Title implements ErrorMember {
     /**
      * @param string $title a short, human-readable summary of the problem that SHOULD NOT change from occurrence
      *                      to occurrence of the problem, except for purposes of localization
      */
-    public function __construct(string $title)
-    {
-        $this->title = $title;
+    public function __construct(private readonly string $title) {
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->title = $this->title;
     }
 }
diff --git a/src/ErrorDocument.php b/src/ErrorDocument.php
index e4a52e5..4cb70b5 100644
--- a/src/ErrorDocument.php
+++ b/src/ErrorDocument.php
@@ -1,28 +1,27 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
+use JsonSerializable;
 
 /**
  * A Document containing an array of Error objects
  * @see http://jsonapi.org/format/#document-top-level
  */
-final class ErrorDocument implements \JsonSerializable
-{
-    private $obj;
+final class ErrorDocument implements JsonSerializable {
+    private readonly object $obj;
 
-    public function __construct(Error $error, ErrorDocumentMember ...$members)
-    {
-        $this->obj = (object) [];
-        $error->attachTo($this->obj);
+    public function __construct(ErrorDocumentMember ...$members) {
+        $this->obj = (object)[];
         foreach ($members as $member) {
             $member->attachTo($this->obj);
         }
     }
 
-    public function jsonSerialize()
-    {
+    public function jsonSerialize(): object {
         return $this->obj;
     }
 }
diff --git a/src/Included.php b/src/Included.php
index 04c8d1c..96a1edb 100644
--- a/src/Included.php
+++ b/src/Included.php
@@ -1,45 +1,37 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Attachable;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
+use LogicException;
 
-final class Included implements Attachable
-{
+/**
+ * A set of included resources.
+ */
+final class Included implements Attachable {
     /**
      * @var ResourceObject[]
      */
-    private $resources = [];
-
-    private $identifiers = [];
+    private array $resources;
 
-    public function __construct(ResourceObject ...$resources)
-    {
+    public function __construct(ResourceObject ...$resources) {
         foreach ($resources as $resource) {
             $key = $resource->key();
             if (isset($this->resources[$key])) {
-                throw new \LogicException("Resource $resource is already included");
+                throw new LogicException("Resource $resource is already included");
             }
             $this->resources[$key] = $resource;
-            $resource->registerIn($this->identifiers);
         }
     }
 
-    public function validateLinkage(PrimaryData $data): void
-    {
-        $registry = [];
-        $data->registerIn($registry);
-        foreach ($this->resources as $resource) {
-            if (isset($registry[$resource->key()]) || isset($this->identifiers[$resource->key()])) {
-                continue;
-            }
-            throw new \LogicException('Full linkage required for '.$resource);
-        }
-    }
-
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         foreach ($this->resources as $resource) {
             $resource->attachAsIncludedTo($o);
         }
diff --git a/src/Internal/Attachable.php b/src/Internal/Attachable.php
index cccd352..01de494 100644
--- a/src/Internal/Attachable.php
+++ b/src/Internal/Attachable.php
@@ -1,11 +1,17 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface Attachable
-{
+interface Attachable {
+    /**
+     * Adds this object's data to $o
+     * @param object $o
+     * @internal
+     */
     public function attachTo(object $o): void;
 }
diff --git a/src/Internal/BaseResource.php b/src/Internal/BaseResource.php
new file mode 100644
index 0000000..b3994a5
--- /dev/null
+++ b/src/Internal/BaseResource.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Internal;
+
+use DomainException;
+use LogicException;
+
+use function JsonApiPhp\JsonApi\isValidName;
+
+/**
+ * Class BaseResource
+ * @internal
+ */
+class BaseResource implements Attachable {
+    protected object $obj;
+    protected array $registry = [];
+
+    public function __construct(protected string $type, ResourceMember ...$members) {
+        if (isValidName($type) === false) {
+            throw new DomainException("Invalid type value: $type");
+        }
+        $this->obj = (object)['type' => $type];
+
+        $this->addMembers(...$members);
+    }
+
+    /**
+     * @param ResourceMember ...$members
+     * @internal
+     */
+    protected function addMembers(ResourceMember ...$members): void {
+        $fields = [];
+        foreach ($members as $member) {
+            if ($member instanceof ResourceField) {
+                $name = $member->name();
+                if (isset($fields[$name])) {
+                    throw new LogicException("Field '$name' already exists'");
+                }
+                $fields[$name] = true;
+            }
+            $member->attachTo($this->obj);
+        }
+    }
+
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
+        $o->data = $this->obj;
+    }
+}
diff --git a/src/Internal/Collection.php b/src/Internal/Collection.php
index e0e73d0..cd8b8d6 100644
--- a/src/Internal/Collection.php
+++ b/src/Internal/Collection.php
@@ -1,7 +1,8 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
-interface Collection extends PrimaryData
-{
+interface Collection extends PrimaryData {
 }
diff --git a/src/Internal/DataDocumentMember.php b/src/Internal/DataDocumentMember.php
index 717ae5e..d69dedb 100644
--- a/src/Internal/DataDocumentMember.php
+++ b/src/Internal/DataDocumentMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface DataDocumentMember extends Attachable
-{
+interface DataDocumentMember extends Attachable {
 }
diff --git a/src/Internal/ErrorDocumentMember.php b/src/Internal/ErrorDocumentMember.php
index b0d01c1..3b5449a 100644
--- a/src/Internal/ErrorDocumentMember.php
+++ b/src/Internal/ErrorDocumentMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface ErrorDocumentMember extends Attachable
-{
+interface ErrorDocumentMember extends Attachable {
 }
diff --git a/src/Internal/ErrorMember.php b/src/Internal/ErrorMember.php
index 0ea2c5d..f115fa2 100644
--- a/src/Internal/ErrorMember.php
+++ b/src/Internal/ErrorMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface ErrorMember extends Attachable
-{
+interface ErrorMember extends Attachable {
 }
diff --git a/src/Internal/Identifier.php b/src/Internal/Identifier.php
index fa435a1..fdfcd87 100644
--- a/src/Internal/Identifier.php
+++ b/src/Internal/Identifier.php
@@ -1,15 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface Identifier
-{
-    /**
-     * @internal
-     * @param array $registry
-     */
-    public function registerIn(array &$registry): void;
+interface Identifier {
 }
diff --git a/src/Internal/LinkTrait.php b/src/Internal/LinkTrait.php
index 006c61a..c4d438b 100644
--- a/src/Internal/LinkTrait.php
+++ b/src/Internal/LinkTrait.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
@@ -7,17 +9,12 @@
 /**
  * @internal
  */
-trait LinkTrait
-{
-    /**
-     * @var string|object
-     */
-    private $link;
+trait LinkTrait {
+    private string|object $link;
 
-    public function __construct(string $url, Meta ...$metas)
-    {
+    public function __construct(string $url, Meta ...$metas) {
         if ($metas) {
-            $this->link = (object) ['href' => $url];
+            $this->link = (object)['href' => $url];
             foreach ($metas as $meta) {
                 $meta->attachTo($this->link);
             }
diff --git a/src/Internal/MetaDocumentMember.php b/src/Internal/MetaDocumentMember.php
index daff35c..e204a70 100644
--- a/src/Internal/MetaDocumentMember.php
+++ b/src/Internal/MetaDocumentMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface MetaDocumentMember
-{
+interface MetaDocumentMember extends Attachable {
 }
diff --git a/src/Internal/PaginationLink.php b/src/Internal/PaginationLink.php
index 00727d5..32c65e6 100644
--- a/src/Internal/PaginationLink.php
+++ b/src/Internal/PaginationLink.php
@@ -1,8 +1,8 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
-interface PaginationLink extends Attachable
-{
+interface PaginationLink extends Attachable {
 }
diff --git a/src/Internal/PrimaryData.php b/src/Internal/PrimaryData.php
index aeeb7ad..c014270 100644
--- a/src/Internal/PrimaryData.php
+++ b/src/Internal/PrimaryData.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface PrimaryData extends Attachable, Identifier
-{
+interface PrimaryData extends Attachable {
 }
diff --git a/src/Internal/RelationshipMember.php b/src/Internal/RelationshipMember.php
new file mode 100644
index 0000000..0a266aa
--- /dev/null
+++ b/src/Internal/RelationshipMember.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Internal;
+
+/**
+ * @internal
+ */
+interface RelationshipMember extends ToOneMember, ToManyMember {
+}
diff --git a/src/Internal/ResourceField.php b/src/Internal/ResourceField.php
index 2d68634..edb6702 100644
--- a/src/Internal/ResourceField.php
+++ b/src/Internal/ResourceField.php
@@ -1,9 +1,9 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
-interface ResourceField extends ResourceMember
-{
+interface ResourceField extends ResourceMember {
     public function name(): string;
 }
diff --git a/src/Internal/ResourceFieldTrait.php b/src/Internal/ResourceFieldTrait.php
index 2d35f3a..5551e63 100644
--- a/src/Internal/ResourceFieldTrait.php
+++ b/src/Internal/ResourceFieldTrait.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
@@ -7,22 +9,19 @@
 /**
  * @internal
  */
-trait ResourceFieldTrait
-{
-    private $name;
+trait ResourceFieldTrait {
+    private readonly string $name;
 
-    private function validateFieldName(string $name): void
-    {
-        if (isValidName($name) === false) {
-            throw new \DomainException("Invalid character in a member name '$name'");
+    private function validateFieldName(string $fieldName): void {
+        if (isValidName($fieldName) === false) {
+            throw new \DomainException("Invalid character in a member name '$fieldName'");
         }
-        if ($name === 'id' || $name === 'type') {
-            throw new \DomainException("Can not use '$name' as a resource field");
+        if ($fieldName === 'id' || $fieldName === 'type') {
+            throw new \DomainException("Can not use '$fieldName' as a resource field");
         }
     }
 
-    public function name(): string
-    {
+    public function name(): string {
         return $this->name;
     }
 }
diff --git a/src/Internal/ResourceMember.php b/src/Internal/ResourceMember.php
index b20e3b0..ba4918d 100644
--- a/src/Internal/ResourceMember.php
+++ b/src/Internal/ResourceMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface ResourceMember extends Attachable
-{
+interface ResourceMember extends Attachable {
 }
diff --git a/src/Internal/ToManyMember.php b/src/Internal/ToManyMember.php
index 52dd2f7..7c5cdc6 100644
--- a/src/Internal/ToManyMember.php
+++ b/src/Internal/ToManyMember.php
@@ -1,7 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
-interface ToManyMember extends ToOneMember
-{
+/**
+ * @internal
+ */
+interface ToManyMember extends Attachable {
 }
diff --git a/src/Internal/ToOneMember.php b/src/Internal/ToOneMember.php
index f65acd4..c1e7a96 100644
--- a/src/Internal/ToOneMember.php
+++ b/src/Internal/ToOneMember.php
@@ -1,10 +1,11 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
 /**
  * @internal
  */
-interface ToOneMember extends Attachable
-{
+interface ToOneMember extends Attachable {
 }
diff --git a/src/JsonApi.php b/src/JsonApi.php
index c0f2220..ac881b2 100644
--- a/src/JsonApi.php
+++ b/src/JsonApi.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
@@ -6,22 +8,21 @@
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
 use JsonApiPhp\JsonApi\Internal\MetaDocumentMember;
 
-final class JsonApi implements MetaDocumentMember, DataDocumentMember, ErrorDocumentMember
-{
-    private $obj;
+final class JsonApi implements MetaDocumentMember, DataDocumentMember, ErrorDocumentMember {
+    private readonly object $obj;
 
-    public function __construct(string $version = '1.0', Meta $meta = null)
-    {
-        $this->obj = (object) [
+    public function __construct(string $version = '1.0', Meta $meta = null) {
+        $this->obj = (object)[
             'version' => $version,
         ];
-        if ($meta) {
-            $meta->attachTo($this->obj);
-        }
+        $meta?->attachTo($this->obj);
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->jsonapi = $this->obj;
     }
 }
diff --git a/src/Link/AboutLink.php b/src/Link/AboutLink.php
index 31fe133..fdafefd 100644
--- a/src/Link/AboutLink.php
+++ b/src/Link/AboutLink.php
@@ -1,17 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class AboutLink implements ErrorMember
-{
+final class AboutLink implements ErrorMember {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->about = $this->link;
     }
 }
diff --git a/src/Link/FirstLink.php b/src/Link/FirstLink.php
index f41ab3c..dfc5b6a 100644
--- a/src/Link/FirstLink.php
+++ b/src/Link/FirstLink.php
@@ -1,17 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class FirstLink implements PaginationLink
-{
+final class FirstLink implements PaginationLink {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->first = $this->link;
     }
 }
diff --git a/src/Link/LastLink.php b/src/Link/LastLink.php
index b174cb2..a2215b8 100644
--- a/src/Link/LastLink.php
+++ b/src/Link/LastLink.php
@@ -1,17 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class LastLink implements PaginationLink
-{
+final class LastLink implements PaginationLink {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->last = $this->link;
     }
 }
diff --git a/src/Link/NextLink.php b/src/Link/NextLink.php
index e1cce27..082fbcc 100644
--- a/src/Link/NextLink.php
+++ b/src/Link/NextLink.php
@@ -1,17 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class NextLink implements PaginationLink
-{
+final class NextLink implements PaginationLink {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->next = $this->link;
     }
 }
diff --git a/src/Link/PrevLink.php b/src/Link/PrevLink.php
index 99a61c3..d38d025 100644
--- a/src/Link/PrevLink.php
+++ b/src/Link/PrevLink.php
@@ -1,17 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class PrevLink implements PaginationLink
-{
+final class PrevLink implements PaginationLink {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->prev = $this->link;
     }
 }
diff --git a/src/Link/RelatedLink.php b/src/Link/RelatedLink.php
index c0005ff..501c2ec 100644
--- a/src/Link/RelatedLink.php
+++ b/src/Link/RelatedLink.php
@@ -1,17 +1,23 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class RelatedLink implements ToOneMember
-{
+final class RelatedLink implements RelationshipMember, DataDocumentMember {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->related = $this->link;
     }
 }
diff --git a/src/Link/SelfLink.php b/src/Link/SelfLink.php
index 17d96cd..0168cb1 100644
--- a/src/Link/SelfLink.php
+++ b/src/Link/SelfLink.php
@@ -1,19 +1,24 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
+
 use function JsonApiPhp\JsonApi\child;
 
-final class SelfLink implements DataDocumentMember, ResourceMember, ToOneMember
-{
+final class SelfLink implements DataDocumentMember, ResourceMember, RelationshipMember {
     use LinkTrait;
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         child($o, 'links')->self = $this->link;
     }
 }
diff --git a/src/Meta.php b/src/Meta.php
index e186648..ebec4dc 100644
--- a/src/Meta.php
+++ b/src/Meta.php
@@ -1,33 +1,38 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
+use DomainException;
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use JsonApiPhp\JsonApi\Internal\MetaDocumentMember;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
-
-final class Meta implements ErrorMember, ErrorDocumentMember, MetaDocumentMember, DataDocumentMember, ResourceMember, ToOneMember
-{
-    /**
-     * @var string
-     */
-    private $key;
-    private $value;
 
-    public function __construct(string $key, $value)
-    {
-        if (isValidName($key) === false) {
-            throw new \DomainException("Invalid character in a member name '$key'");
+final class Meta implements
+    ErrorMember,
+    ErrorDocumentMember,
+    MetaDocumentMember,
+    DataDocumentMember,
+    ResourceMember,
+    RelationshipMember {
+        public function __construct(
+            private readonly string $key,
+            private readonly string|int|float|bool|null|array|object $value
+        ) {
+            if (!isValidName($key)) {
+                throw new DomainException("Invalid character in a member name '$key'");
+            }
         }
-        $this->key = $key;
-        $this->value = $value;
-    }
 
-    public function attachTo(object $o): void
-    {
-        child($o, 'meta')->{$this->key} = $this->value;
+        /**
+         * @param object $o
+         * @internal
+         */
+        public function attachTo(object $o): void {
+            child($o, 'meta')->{$this->key} = $this->value;
+        }
     }
-}
diff --git a/src/MetaDocument.php b/src/MetaDocument.php
index 72ca320..14f0334 100644
--- a/src/MetaDocument.php
+++ b/src/MetaDocument.php
@@ -1,20 +1,20 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\MetaDocumentMember;
+use JsonSerializable;
 
-final class MetaDocument implements \JsonSerializable
-{
-    private $doc;
+final class MetaDocument implements JsonSerializable {
+    private readonly object $doc;
 
-    public function __construct(Meta $meta, MetaDocumentMember ...$members)
-    {
+    public function __construct(Meta $meta, MetaDocumentMember ...$members) {
         $this->doc = combine($meta, ...$members);
     }
 
-    public function jsonSerialize()
-    {
+    public function jsonSerialize(): object {
         return $this->doc;
     }
 }
diff --git a/src/NewResourceObject.php b/src/NewResourceObject.php
new file mode 100644
index 0000000..44364f1
--- /dev/null
+++ b/src/NewResourceObject.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi;
+
+use JsonApiPhp\JsonApi\Internal\BaseResource;
+use JsonApiPhp\JsonApi\Internal\PrimaryData;
+use JsonApiPhp\JsonApi\Internal\ResourceMember;
+
+/**
+ * A resource to-be-created on the server. Does not have the `id` yet.
+ *
+ * Class NewResourceObject
+ */
+final class NewResourceObject extends BaseResource implements PrimaryData {
+    public function __construct(string $type, ResourceMember ...$members) {
+        parent::__construct($type, ...$members);
+    }
+}
diff --git a/src/NullData.php b/src/NullData.php
index 26dd8a5..5f65be6 100644
--- a/src/NullData.php
+++ b/src/NullData.php
@@ -1,17 +1,17 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-final class NullData implements PrimaryData
-{
-    public function attachTo(object $o): void
-    {
+final class NullData implements PrimaryData {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->data = null;
     }
-
-    public function registerIn(array &$registry): void
-    {
-    }
 }
diff --git a/src/PaginatedCollection.php b/src/PaginatedCollection.php
index 8b8e0bd..ffc9050 100644
--- a/src/PaginatedCollection.php
+++ b/src/PaginatedCollection.php
@@ -1,39 +1,22 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Collection;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-final class PaginatedCollection implements PrimaryData
-{
-    /**
-     * @var Pagination
-     */
-    private $pagination;
-    /**
-     * @var Collection
-     */
-    private $collection;
-
-    public function __construct(Pagination $pagination, Collection $collection)
-    {
-        $this->pagination = $pagination;
-        $this->collection = $collection;
-    }
-
-    public function attachTo(object $o): void
-    {
-        $this->collection->attachTo($o);
-        $this->pagination->attachTo($o);
+final class PaginatedCollection implements PrimaryData {
+    public function __construct(private readonly Pagination $pagination, private readonly Collection $collection) {
     }
 
     /**
+     * @param object $o
      * @internal
-     * @param array $registry
      */
-    public function registerIn(array &$registry): void
-    {
-        $this->collection->registerIn($registry);
+    public function attachTo(object $o): void {
+        $this->collection->attachTo($o);
+        $this->pagination->attachTo($o);
     }
 }
diff --git a/src/Pagination.php b/src/Pagination.php
index aa4f9c3..dc2c642 100644
--- a/src/Pagination.php
+++ b/src/Pagination.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
@@ -6,20 +7,21 @@
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
 use JsonApiPhp\JsonApi\Internal\ToManyMember;
 
-class Pagination implements ToManyMember
-{
+class Pagination implements ToManyMember {
     /**
      * @var PaginationLink[]
      */
-    private $links;
+    private readonly array $links;
 
-    public function __construct(PaginationLink ...$links)
-    {
+    public function __construct(PaginationLink ...$links) {
         $this->links = $links;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         foreach ($this->links as $link) {
             $link->attachTo($o);
         }
diff --git a/src/ResourceCollection.php b/src/ResourceCollection.php
index 77ac09f..592dd21 100644
--- a/src/ResourceCollection.php
+++ b/src/ResourceCollection.php
@@ -1,34 +1,30 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Collection;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-class ResourceCollection implements PrimaryData, Collection
-{
+class ResourceCollection implements PrimaryData, Collection {
     /**
      * @var ResourceObject[]
      */
-    private $resources;
+    private readonly array $resources;
 
-    public function __construct(ResourceObject ...$resources)
-    {
+    public function __construct(ResourceObject ...$resources) {
         $this->resources = $resources;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->data = [];
         foreach ($this->resources as $resource) {
             $resource->attachToCollection($o);
         }
     }
-
-    public function registerIn(array &$registry): void
-    {
-        foreach ($this->resources as $resource) {
-            $resource->registerIn($registry);
-        }
-    }
 }
diff --git a/src/ResourceIdentifier.php b/src/ResourceIdentifier.php
index 221e096..cf1cf88 100644
--- a/src/ResourceIdentifier.php
+++ b/src/ResourceIdentifier.php
@@ -1,50 +1,41 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
+use DomainException;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-final class ResourceIdentifier implements PrimaryData
-{
-    private $obj;
-    /**
-     * @var string
-     */
-    private $type;
-    /**
-     * @var string
-     */
-    private $id;
+final class ResourceIdentifier implements PrimaryData {
+    private readonly object $obj;
 
-    public function __construct(string $type, string $id, Meta $meta = null)
-    {
+    public function __construct(string $type, string $id, Meta ...$metas) {
         if (isValidName($type) === false) {
-            throw new \DomainException("Invalid type value: $type");
+            throw new DomainException("Invalid type value: $type");
         }
-
-        $this->obj = (object) [
+        $this->obj = (object)[
             'type' => $type,
             'id' => $id,
         ];
-        if ($meta) {
+        foreach ($metas as $meta) {
             $meta->attachTo($this->obj);
         }
-        $this->type = $type;
-        $this->id = $id;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->data = $this->obj;
     }
 
-    public function attachToCollection(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachToCollection(object $o): void {
         $o->data[] = $this->obj;
     }
-
-    public function registerIn(array &$registry): void
-    {
-        $registry[compositeKey($this->type, $this->id)] = true;
-    }
 }
diff --git a/src/ResourceIdentifierCollection.php b/src/ResourceIdentifierCollection.php
index d13eb11..794789e 100644
--- a/src/ResourceIdentifierCollection.php
+++ b/src/ResourceIdentifierCollection.php
@@ -1,34 +1,30 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Collection;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-class ResourceIdentifierCollection implements PrimaryData, Collection
-{
+class ResourceIdentifierCollection implements PrimaryData, Collection {
     /**
      * @var ResourceIdentifier[]
      */
-    private $identifiers = [];
+    private readonly array $identifiers;
 
-    public function __construct(ResourceIdentifier ...$identifiers)
-    {
+    public function __construct(ResourceIdentifier ...$identifiers) {
         $this->identifiers = $identifiers;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->data = [];
         foreach ($this->identifiers as $identifier) {
             $identifier->attachToCollection($o);
         }
     }
-
-    public function registerIn(array &$registry): void
-    {
-        foreach ($this->identifiers as $identifier) {
-            $identifier->registerIn($registry);
-        }
-    }
 }
diff --git a/src/ResourceObject.php b/src/ResourceObject.php
index de0a478..f65d575 100644
--- a/src/ResourceObject.php
+++ b/src/ResourceObject.php
@@ -1,81 +1,52 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\Identifier;
+use JsonApiPhp\JsonApi\Internal\BaseResource;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
-use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
 
-final class ResourceObject implements PrimaryData
-{
-    private $obj;
-    private $registry = [];
-    /**
-     * @var string
-     */
-    private $type;
-    /**
-     * @var string
-     */
-    private $id;
-
-    public function __construct(string $type, string $id, ResourceMember ...$members)
-    {
-        if (isValidName($type) === false) {
-            throw new \DomainException("Invalid type value: $type");
-        }
-        $this->obj = (object) ['type' => $type, 'id' => $id];
-        $fields = [];
-        foreach ($members as $member) {
-            if ($member instanceof Identifier) {
-                $member->registerIn($this->registry);
-            }
-            if ($member instanceof ResourceField) {
-                $name = $member->name();
-                if (isset($fields[$name])) {
-                    throw new \LogicException("Field '$name' already exists'");
-                }
-                $fields[$name] = true;
-            }
-            $member->attachTo($this->obj);
-        }
+final class ResourceObject extends BaseResource implements PrimaryData {
+    public function __construct(string $type, private readonly string $id, ResourceMember ...$members) {
+        parent::__construct($type, ...$members);
+        $this->obj->id = $id;
         $this->type = $type;
-        $this->id = $id;
     }
 
-    public function identifier(): ResourceIdentifier
-    {
+    public function identifier(): ResourceIdentifier {
         return new ResourceIdentifier($this->type, $this->id);
     }
 
-    public function key(): string
-    {
+    public function key(): string {
         return compositeKey($this->type, $this->id);
     }
 
-    public function registerIn(array &$registry): void
-    {
-        $registry = array_merge($registry, $this->registry);
-    }
-
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $o->data = $this->obj;
     }
 
-    public function attachAsIncludedTo(object $o): void
-    {
+    /**
+     * @param object $o
+     */
+    public function attachAsIncludedTo(object $o): void {
         $o->included[] = $this->obj;
     }
 
-    public function attachToCollection(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachToCollection(object $o): void {
         $o->data[] = $this->obj;
     }
 
-    public function __toString(): string
-    {
+    public function __toString(): string {
         return $this->key();
     }
 }
diff --git a/src/ToMany.php b/src/ToMany.php
index 94b873b..afe4987 100644
--- a/src/ToMany.php
+++ b/src/ToMany.php
@@ -1,34 +1,37 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Identifier;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
+use JsonApiPhp\JsonApi\Internal\ToManyMember;
 
-final class ToMany implements Identifier, ResourceField
-{
+final class ToMany implements Identifier, ResourceField {
     use ResourceFieldTrait;
+
     /**
-     * @var ToOneMember[]
-     */
-    private $members;
-    /**
-     * @var ResourceIdentifierCollection
+     * @var ToManyMember[]
      */
-    private $collection;
+    private readonly array $members;
 
-    public function __construct(string $name, ResourceIdentifierCollection $collection, ToOneMember ...$members)
-    {
+    public function __construct(
+        string $name,
+        private readonly ResourceIdentifierCollection $collection,
+        ToManyMember ...$members
+    ) {
         $this->validateFieldName($name);
         $this->name = $name;
         $this->members = $members;
-        $this->collection = $collection;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $rel = child(child($o, 'relationships'), $this->name);
         $rel->data = [];
         $this->collection->attachTo($rel);
@@ -36,9 +39,4 @@ public function attachTo(object $o): void
             $member->attachTo($rel);
         }
     }
-
-    public function registerIn(array &$registry): void
-    {
-        $this->collection->registerIn($registry);
-    }
 }
diff --git a/src/ToNull.php b/src/ToNull.php
index 0efaef0..cbaec93 100644
--- a/src/ToNull.php
+++ b/src/ToNull.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
@@ -6,23 +8,25 @@
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
 use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
-final class ToNull implements ResourceField
-{
+final class ToNull implements ResourceField {
     use ResourceFieldTrait;
+
     /**
      * @var ToOneMember[]
      */
-    private $members;
+    private readonly array $members;
 
-    public function __construct(string $name, ToOneMember ...$members)
-    {
+    public function __construct(string $name, ToOneMember ...$members) {
         $this->validateFieldName($name);
         $this->name = $name;
         $this->members = $members;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo(object $o): void {
         $obj = combine(...$this->members);
         $obj->data = null;
         child($o, 'relationships')->{$this->name} = $obj;
diff --git a/src/ToOne.php b/src/ToOne.php
index c6d32da..eb86853 100644
--- a/src/ToOne.php
+++ b/src/ToOne.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
@@ -7,32 +9,22 @@
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
 use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
-final class ToOne implements Identifier, ResourceField
-{
+final class ToOne implements Identifier, ResourceField {
     use ResourceFieldTrait;
 
-    /**
-     * @var ResourceIdentifier
-     */
-    private $identifier;
-
-    private $obj;
+    private readonly object $obj;
 
-    public function __construct(string $name, ResourceIdentifier $identifier, ToOneMember ...$members)
-    {
+    public function __construct(string $name, private readonly ResourceIdentifier $identifier, ToOneMember ...$members) {
         $this->validateFieldName($name);
         $this->name = $name;
         $this->obj = combine($identifier, ...$members);
-        $this->identifier = $identifier;
     }
 
-    public function attachTo(object $o): void
-    {
+    /**
+     * @param object $o
+     * @internal
+     */
+    public function attachTo($o): void {
         child($o, 'relationships')->{$this->name} = $this->obj;
     }
-
-    public function registerIn(array &$registry): void
-    {
-        $this->identifier->registerIn($registry);
-    }
 }
diff --git a/src/functions.php b/src/functions.php
index d7c89a4..5a08291 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -1,32 +1,30 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Attachable;
 
-function combine(Attachable ...$members): object
-{
-    $obj = (object) [];
+function combine(Attachable ...$members): object {
+    $obj = (object)[];
     foreach ($members as $member) {
         $member->attachTo($obj);
     }
     return $obj;
 }
 
-function child(object $o, string $name): object
-{
+function child($o, string $name): mixed {
     if (!isset($o->{$name})) {
-        $o->{$name} = (object) [];
+        $o->{$name} = (object)[];
     }
     return $o->{$name};
 }
 
-function isValidName(string $name): bool
-{
+function isValidName(string $name): bool {
     return preg_match('/^(?=[^-_ ])[a-zA-Z0-9\x{0080}-\x{FFFF}-_ ]*(?<=[^-_ ])$/u', $name) === 1;
 }
 
-function compositeKey(string $type, string $id): string
-{
+function compositeKey(string $type, string $id): string {
     return "{$type}:{$id}";
 }
diff --git a/test/BaseTestCase.php b/test/BaseTestCase.php
index 2485e47..e649b2c 100644
--- a/test/BaseTestCase.php
+++ b/test/BaseTestCase.php
@@ -1,13 +1,13 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
 use PHPUnit\Framework\TestCase;
 
-abstract class BaseTestCase extends TestCase
-{
-    public static function assertEncodesTo(string $expected, object $obj, string $message = '')
-    {
+abstract class BaseTestCase extends TestCase {
+    public static function assertEncodesTo(string $expected, $obj, string $message = '') {
         self::assertEquals(
             json_decode($expected),
             json_decode(json_encode($obj, JSON_UNESCAPED_SLASHES)),
diff --git a/test/CompoundDocumentTest.php b/test/CompoundDocumentTest.php
index 3d62ff9..cb20ac6 100644
--- a/test/CompoundDocumentTest.php
+++ b/test/CompoundDocumentTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
@@ -19,10 +21,8 @@
 use JsonApiPhp\JsonApi\ToMany;
 use JsonApiPhp\JsonApi\ToOne;
 
-class CompoundDocumentTest extends BaseTestCase
-{
-    public function testOfficialDocsExample()
-    {
+class CompoundDocumentTest extends BaseTestCase {
+    public function testOfficialDocsExample() {
         $dan = new ResourceObject(
             'people',
             '9',
@@ -38,8 +38,8 @@ public function testOfficialDocsExample()
             new Attribute('body', 'First!'),
             new SelfLink('http://example.com/comments/5'),
             new ToOne('author', new ResourceIdentifier('people', '2'))
-
         );
+
         $comment12 = new ResourceObject(
             'comments',
             '12',
@@ -164,65 +164,7 @@ public function testOfficialDocsExample()
         );
     }
 
-    /**
-     * Compound documents require “full linkage”, meaning that every included resource MUST be identified
-     * by at least one resource identifier object in the same document.
-     * These resource identifier objects could either be primary data or represent resource linkage
-     * contained within primary or included resources.
-     *
-     * @dataProvider documentsWithoutFullLinkage
-     * @param callable $create_doc
-     */
-    public function testFullLinkage(callable $create_doc)
-    {
-        $this->expectException(\LogicException::class);
-        $this->expectExceptionMessage('Full linkage required for apples:1');
-        $create_doc();
-    }
-
-    public function documentsWithoutFullLinkage(): array
-    {
-        $included = new Included(new ResourceObject('apples', '1'));
-        return [
-            [
-                function () use ($included) {
-                    return new CompoundDocument(new NullData(), $included);
-                },
-            ],
-            [
-                function () use ($included) {
-                    return new CompoundDocument(new ResourceCollection(), $included);
-                },
-            ],
-            [
-                function () use ($included) {
-                    return new CompoundDocument(new ResourceIdentifier('oranges', '1'), $included);
-                },
-            ],
-            [
-                function () use ($included) {
-                    return new CompoundDocument(
-                        new ResourceIdentifierCollection(
-                            new ResourceIdentifier('oranges', '1'),
-                            new ResourceIdentifier('oranges', '1')
-                        ),
-                        $included
-                    );
-                },
-            ],
-            [
-                function () use ($included) {
-                    return new CompoundDocument(
-                        new ResourceCollection(new ResourceObject('oranges', '1'), new ResourceObject('oranges', '1')),
-                        $included
-                    );
-                },
-            ],
-        ];
-    }
-
-    public function testIncludedResourceMayBeIdentifiedByLinkageInPrimaryData()
-    {
+    public function testIncludedResourceMayBeIdentifiedByLinkageInPrimaryData() {
         $author = new ResourceObject('people', '9');
         $article = new ResourceObject(
             'articles',
@@ -233,8 +175,7 @@ public function testIncludedResourceMayBeIdentifiedByLinkageInPrimaryData()
         $this->assertNotEmpty($doc);
     }
 
-    public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
-    {
+    public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource() {
         $writer = new ResourceObject('writers', '3', new Attribute('name', 'Eric Evans'));
         $book = new ResourceObject(
             'books',
@@ -253,11 +194,10 @@ public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
 
     /**
      * A compound document MUST NOT include more than one resource object for each type and id pair.
-     * @expectedException \LogicException
-     * @expectedExceptionMessage Resource apples:1 is already included
      */
-    public function testCanNotBeManyIncludedResourcesWithEqualIdentifiers()
-    {
+    public function testCanNotBeManyIncludedResourcesWithEqualIdentifiers() {
+        $this->expectException('LogicException');
+        $this->expectExceptionMessage('Resource apples:1 is already included');
         $apple = new ResourceObject('apples', '1');
         new CompoundDocument($apple->identifier(), new Included($apple, $apple));
     }
diff --git a/test/DataDocument/ManyResourceIdentifiersTest.php b/test/DataDocument/ManyResourceIdentifiersTest.php
index eb12973..2d4728d 100644
--- a/test/DataDocument/ManyResourceIdentifiersTest.php
+++ b/test/DataDocument/ManyResourceIdentifiersTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test\DataDocument;
 
@@ -10,10 +12,8 @@
 use JsonApiPhp\JsonApi\ResourceIdentifierCollection;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
-class ManyResourceIdentifiersTest extends BaseTestCase
-{
-    public function testMinimalDocument()
-    {
+class ManyResourceIdentifiersTest extends BaseTestCase {
+    public function testMinimalDocument() {
         $this->assertEncodesTo(
             '
             {
@@ -26,8 +26,7 @@ public function testMinimalDocument()
         );
     }
 
-    public function testExtendedDocument()
-    {
+    public function testExtendedDocument() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/DataDocument/ManyResourceObjectsTest.php b/test/DataDocument/ManyResourceObjectsTest.php
index 3a2be82..a555e0c 100644
--- a/test/DataDocument/ManyResourceObjectsTest.php
+++ b/test/DataDocument/ManyResourceObjectsTest.php
@@ -1,20 +1,21 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test\DataDocument;
 
 use JsonApiPhp\JsonApi\Attribute;
 use JsonApiPhp\JsonApi\DataDocument;
 use JsonApiPhp\JsonApi\JsonApi;
+use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\Meta;
 use JsonApiPhp\JsonApi\ResourceCollection;
 use JsonApiPhp\JsonApi\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
-class ManyResourceObjectsTest extends BaseTestCase
-{
-    public function testMinimalDocument()
-    {
+class ManyResourceObjectsTest extends BaseTestCase {
+    public function testMinimalDocument() {
         $this->assertEncodesTo(
             '
             {
@@ -27,30 +28,28 @@ public function testMinimalDocument()
         );
     }
 
-    public function testExtendedDocument()
-    {
+    public function testExtendedDocument() {
         $this->assertEncodesTo(
             '
             {
                 "data": [{
-                    "type": "apples",
+                    "type": "people",
                     "id": "1",
                     "attributes": {
-                        "color": "red",
-                        "sort": "Fuji"
+                        "name": "Martin Fowler"
                     },
                     "meta": {"apple_meta": "foo"}
                 },{
-                    "type": "apples",
+                    "type": "people",
                     "id": "2",
                     "attributes": {
-                        "color": "yellow",
-                        "sort": "Gala"
+                        "name": "Kent Beck"
                     },
                     "meta": {"apple_meta": "foo"}
                 }],
                 "links": {
-                    "self": "/apples"
+                    "self": "/books/123/relationships/authors",
+                    "related": "/books/123/authors"
                 },
                 "jsonapi": {
                     "version": "1.0"
@@ -61,21 +60,20 @@ public function testExtendedDocument()
             new DataDocument(
                 new ResourceCollection(
                     new ResourceObject(
-                        'apples',
+                        'people',
                         '1',
-                        new Attribute('color', 'red'),
-                        new Attribute('sort', 'Fuji'),
+                        new Attribute('name', 'Martin Fowler'),
                         new Meta('apple_meta', 'foo')
                     ),
                     new ResourceObject(
-                        'apples',
+                        'people',
                         '2',
-                        new Attribute('color', 'yellow'),
-                        new Attribute('sort', 'Gala'),
+                        new Attribute('name', 'Kent Beck'),
                         new Meta('apple_meta', 'foo')
                     )
                 ),
-                new SelfLink('/apples'),
+                new SelfLink('/books/123/relationships/authors'),
+                new RelatedLink('/books/123/authors'),
                 new JsonApi(),
                 new Meta('document_meta', 'bar')
             )
diff --git a/test/DataDocument/NullDataTest.php b/test/DataDocument/NullDataTest.php
index 05746cd..4bb759d 100644
--- a/test/DataDocument/NullDataTest.php
+++ b/test/DataDocument/NullDataTest.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test\DataDocument;
@@ -10,10 +11,8 @@
 use JsonApiPhp\JsonApi\NullData;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
-class NullDataTest extends BaseTestCase
-{
-    public function testMinimalDocument()
-    {
+class NullDataTest extends BaseTestCase {
+    public function testMinimalDocument() {
         $this->assertEncodesTo(
             '
             {
@@ -26,8 +25,7 @@ public function testMinimalDocument()
         );
     }
 
-    public function testExtendedDocument()
-    {
+    public function testExtendedDocument() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/DataDocument/SingleResourceIdentifierTest.php b/test/DataDocument/SingleResourceIdentifierTest.php
index 92a91d5..3eea202 100644
--- a/test/DataDocument/SingleResourceIdentifierTest.php
+++ b/test/DataDocument/SingleResourceIdentifierTest.php
@@ -1,45 +1,49 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test\DataDocument;
 
 use JsonApiPhp\JsonApi\DataDocument;
 use JsonApiPhp\JsonApi\JsonApi;
+use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\Meta;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
-class SingleResourceIdentifierTest extends BaseTestCase
-{
-    public function testMinimalDocument()
-    {
+class SingleResourceIdentifierTest extends BaseTestCase {
+    public function testMinimalDocument() {
         $this->assertEncodesTo(
             '
             {
                 "data": {
-                    "type": "apples",
+                    "type": "companies",
                     "id": "1"
                 }
             }
             ',
             new DataDocument(
-                new ResourceIdentifier('apples', '1')
+                new ResourceIdentifier('companies', '1')
             )
         );
     }
 
-    public function testExtendedDocument()
-    {
+    public function testExtendedDocument() {
         $this->assertEncodesTo(
             '
             {
                 "data": {
-                    "type": "apples",
+                    "type": "companies",
                     "id": "1",
-                    "meta": {"apple_meta": "foo"}
+                    "meta": {
+                        "apple_meta": "foo", 
+                        "bar": [42]
+                    }
                 },
                 "links": {
-                    "self": "/apples/1"
+                    "self": "/books/123/relationships/publisher",
+                    "related": "/books/123/publisher"
                 },
                 "jsonapi": {
                     "version": "1.0"
@@ -49,11 +53,13 @@ public function testExtendedDocument()
             ',
             new DataDocument(
                 new ResourceIdentifier(
-                    'apples',
+                    'companies',
                     '1',
-                    new Meta('apple_meta', 'foo')
+                    new Meta('apple_meta', 'foo'),
+                    new Meta('bar', [42])
                 ),
-                new SelfLink('/apples/1'),
+                new SelfLink('/books/123/relationships/publisher'),
+                new RelatedLink('/books/123/publisher'),
                 new JsonApi(),
                 new Meta('document_meta', 'bar')
             )
diff --git a/test/DataDocument/SingleResourceObjectTest.php b/test/DataDocument/SingleResourceObjectTest.php
index dfa25a1..0d19dde 100644
--- a/test/DataDocument/SingleResourceObjectTest.php
+++ b/test/DataDocument/SingleResourceObjectTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test\DataDocument;
 
@@ -10,10 +12,8 @@
 use JsonApiPhp\JsonApi\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
-class SingleResourceObjectTest extends BaseTestCase
-{
-    public function testMinimalDocument()
-    {
+class SingleResourceObjectTest extends BaseTestCase {
+    public function testMinimalDocument() {
         $this->assertEncodesTo(
             '
             {
@@ -29,8 +29,7 @@ public function testMinimalDocument()
         );
     }
 
-    public function testExtendedDocument()
-    {
+    public function testExtendedDocument() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/ErrorDocumentTest.php b/test/ErrorDocumentTest.php
index 9a9131d..cb0bc4c 100644
--- a/test/ErrorDocumentTest.php
+++ b/test/ErrorDocumentTest.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
@@ -16,14 +17,12 @@
 use JsonApiPhp\JsonApi\Link\AboutLink;
 use JsonApiPhp\JsonApi\Meta;
 
-class ErrorDocumentTest extends BaseTestCase
-{
+class ErrorDocumentTest extends BaseTestCase {
     /**
      * The spec does not define any mandatory error members,
      * so the minimal valid error document contains a single empty object
      */
-    public function testMinimalExample()
-    {
+    public function testMinimalExample() {
         $this->assertEncodesTo(
             '
             {
@@ -36,8 +35,7 @@ public function testMinimalExample()
         );
     }
 
-    public function testExtensiveExample()
-    {
+    public function testExtensiveExample() {
         $this->assertEncodesTo(
             '
             {
@@ -82,8 +80,7 @@ public function testExtensiveExample()
         );
     }
 
-    public function testMultipleErrors()
-    {
+    public function testMultipleErrors() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/ExamplesTest.php b/test/ExamplesTest.php
index ba7a1cf..7dc5da4 100644
--- a/test/ExamplesTest.php
+++ b/test/ExamplesTest.php
@@ -1,23 +1,21 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
 use PHPUnit\Framework\TestCase;
 
-class ExamplesTest extends TestCase
-{
+class ExamplesTest extends TestCase {
     /**
      * @dataProvider examples
      * @param string $file
      */
-    public function testOutputEncodesToJson(string $file)
-    {
+    public function testOutputEncodesToJson(string $file) {
         $this->assertJson(`php $file`);
     }
 
-    public function examples()
-    {
+    public function examples() {
         return [
             [__DIR__.'/../examples/compound_doc.php'],
             [__DIR__.'/../examples/simple_doc.php'],
diff --git a/test/JsonApiTest.php b/test/JsonApiTest.php
index 49c5982..8a5ce91 100644
--- a/test/JsonApiTest.php
+++ b/test/JsonApiTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
@@ -6,10 +8,8 @@
 use JsonApiPhp\JsonApi\Meta;
 use JsonApiPhp\JsonApi\MetaDocument;
 
-class JsonApiTest extends BaseTestCase
-{
-    public function testJsonApiMayContainVersionAndMeta()
-    {
+class JsonApiTest extends BaseTestCase {
+    public function testJsonApiMayContainVersionAndMeta() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/LinkObjectTest.php b/test/LinkObjectTest.php
index a69443c..00a9a78 100644
--- a/test/LinkObjectTest.php
+++ b/test/LinkObjectTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
@@ -7,10 +9,8 @@
 use JsonApiPhp\JsonApi\Meta;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
 
-class LinkObjectTest extends BaseTestCase
-{
-    public function testLinkObject()
-    {
+class LinkObjectTest extends BaseTestCase {
+    public function testLinkObject() {
         $this->assertEncodesTo(
             '{
                 "data": {"type": "apples", "id": "1"},
diff --git a/test/MetaDocumentTest.php b/test/MetaDocumentTest.php
index 7893dae..1837f3e 100644
--- a/test/MetaDocumentTest.php
+++ b/test/MetaDocumentTest.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
@@ -7,13 +8,11 @@
 use JsonApiPhp\JsonApi\Meta;
 use JsonApiPhp\JsonApi\MetaDocument;
 
-class MetaDocumentTest extends BaseTestCase
-{
+class MetaDocumentTest extends BaseTestCase {
     /**
      * A valid document may contain just a meta object
      */
-    public function testMetaDocument()
-    {
+    public function testMetaDocument() {
         $this->assertEncodesTo(
             '
             {
@@ -29,8 +28,7 @@ public function testMetaDocument()
     /**
      * A meta document may contain jsonapi member
      */
-    public function testMetaDocumentWithExtraMembers()
-    {
+    public function testMetaDocumentWithExtraMembers() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/MetaTest.php b/test/MetaTest.php
index 67d3097..c8d8d93 100644
--- a/test/MetaTest.php
+++ b/test/MetaTest.php
@@ -1,14 +1,14 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
 use JsonApiPhp\JsonApi\Meta;
 use PHPUnit\Framework\TestCase;
 
-class MetaTest extends TestCase
-{
-    public function testKeyMustBeValid()
-    {
+class MetaTest extends TestCase {
+    public function testKeyMustBeValid() {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage("Invalid character in a member name 'invalid:name'");
         new Meta('invalid:name', '1');
diff --git a/test/NewResourceObjectTest.php b/test/NewResourceObjectTest.php
new file mode 100644
index 0000000..a2fc94e
--- /dev/null
+++ b/test/NewResourceObjectTest.php
@@ -0,0 +1,201 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Test;
+
+use JsonApiPhp\JsonApi\Attribute;
+use JsonApiPhp\JsonApi\DataDocument;
+use JsonApiPhp\JsonApi\EmptyRelationship;
+use JsonApiPhp\JsonApi\Link\RelatedLink;
+use JsonApiPhp\JsonApi\Link\SelfLink;
+use JsonApiPhp\JsonApi\Meta;
+use JsonApiPhp\JsonApi\NewResourceObject;
+use JsonApiPhp\JsonApi\ResourceIdentifier;
+use JsonApiPhp\JsonApi\ResourceIdentifierCollection;
+use JsonApiPhp\JsonApi\ToMany;
+use JsonApiPhp\JsonApi\ToNull;
+use JsonApiPhp\JsonApi\ToOne;
+
+class NewResourceObjectTest extends BaseTestCase {
+    public function testFullFledgedResourceObject() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "apples",
+                    "attributes": {
+                        "title": "Rails is Omakase"
+                    },
+                    "meta": {"foo": "bar"},
+                    "relationships": {
+                        "author": {
+                            "meta": {"foo": "bar"},
+                            "data": null
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'apples',
+                    new Meta('foo', 'bar'),
+                    new Attribute('title', 'Rails is Omakase'),
+                    new ToNull(
+                        'author',
+                        new Meta('foo', 'bar')
+                    )
+                )
+            )
+        );
+    }
+
+    public function testRelationshipWithSingleIdLinkage() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "relationships": {
+                        "content": {
+                            "data": {"type": "apples", "id": "1"}
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'basket',
+                    new ToOne('content', new ResourceIdentifier('apples', '1'))
+                )
+            )
+        );
+    }
+
+    public function testRelationshipWithMultiIdLinkage() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "relationships": {
+                        "content": {
+                            "data": [{
+                                "type": "apples",
+                                "id": "1"
+                            },{
+                                "type": "pears",
+                                "id": "2"
+                            }]
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'basket',
+                    new ToMany(
+                        'content',
+                        new ResourceIdentifierCollection(
+                            new ResourceIdentifier('apples', '1'),
+                            new ResourceIdentifier('pears', '2')
+                        )
+                    )
+                )
+            )
+        );
+    }
+
+    public function testRelationshipWithEmptyMultiIdLinkage() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "relationships": {
+                        "content": {
+                            "data": []
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'basket',
+                    new ToMany('content', new ResourceIdentifierCollection())
+                )
+            )
+        );
+    }
+
+    public function testRelationshipWithNoData() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'basket',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'))
+                )
+            )
+        );
+
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo",
+                                "self": "/bar"
+                            },
+                            "meta": {
+                                "foo": "bar"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new NewResourceObject(
+                    'basket',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'), new SelfLink('/bar'), new Meta('foo', 'bar'))
+                )
+            )
+        );
+    }
+
+    public function testResourceFieldsMustBeUnique() {
+        $this->expectException(\LogicException::class);
+        $this->expectExceptionMessage("Field 'foo' already exists");
+        new NewResourceObject(
+            'apples',
+            new Attribute('foo', 'bar'),
+            new ToOne('foo', new ResourceIdentifier('apples', '1'))
+        );
+    }
+
+    public function testNameValidation() {
+        $this->expectException(\DomainException::class);
+        new NewResourceObject('invalid:id');
+    }
+}
diff --git a/test/PaginationTest.php b/test/PaginationTest.php
index 17ee668..a9307d8 100644
--- a/test/PaginationTest.php
+++ b/test/PaginationTest.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
@@ -10,6 +12,7 @@
 use JsonApiPhp\JsonApi\Link\LastLink;
 use JsonApiPhp\JsonApi\Link\NextLink;
 use JsonApiPhp\JsonApi\Link\PrevLink;
+use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\PaginatedCollection;
 use JsonApiPhp\JsonApi\Pagination;
 use JsonApiPhp\JsonApi\ResourceCollection;
@@ -18,10 +21,8 @@
 use JsonApiPhp\JsonApi\ResourceObject;
 use JsonApiPhp\JsonApi\ToMany;
 
-class PaginationTest extends BaseTestCase
-{
-    public function testPaginatedResourceCollection()
-    {
+class PaginationTest extends BaseTestCase {
+    public function testPaginatedResourceCollection() {
         $this->assertEncodesTo(
             '
             {
@@ -33,7 +34,8 @@ public function testPaginatedResourceCollection()
                     "first": "http://example.com/fruits?page=first",
                     "last": "http://example.com/fruits?page=last",
                     "prev": "http://example.com/fruits?page=3",
-                    "next": "http://example.com/fruits?page=5"
+                    "next": "http://example.com/fruits?page=5",
+                    "self": "http://example.com/fruits?page=4"
                 }
             }
             ',
@@ -49,13 +51,13 @@ public function testPaginatedResourceCollection()
                         new ResourceObject('apples', '1'),
                         new ResourceObject('apples', '2')
                     )
-                )
+                ),
+                new SelfLink('http://example.com/fruits?page=4')
             )
         );
     }
 
-    public function testPaginatedResourceIdentifierCollection()
-    {
+    public function testPaginatedResourceIdentifierCollection() {
         $this->assertEncodesTo(
             '
             {
diff --git a/test/ResourceIdentifierTest.php b/test/ResourceIdentifierTest.php
index 5dcd959..eb479ee 100644
--- a/test/ResourceIdentifierTest.php
+++ b/test/ResourceIdentifierTest.php
@@ -1,14 +1,13 @@
 <?php
+
 declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
 use JsonApiPhp\JsonApi\ResourceIdentifier;
 
-class ResourceIdentifierTest extends BaseTestCase
-{
-    public function testNameValidation()
-    {
+class ResourceIdentifierTest extends BaseTestCase {
+    public function testNameValidation() {
         $this->expectException(\DomainException::class);
         new ResourceIdentifier('invalid:id', 'foo');
     }
diff --git a/test/ResourceObjectTest.php b/test/ResourceObjectTest.php
index 37e845f..b565d7d 100644
--- a/test/ResourceObjectTest.php
+++ b/test/ResourceObjectTest.php
@@ -1,9 +1,12 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Test;
 
 use JsonApiPhp\JsonApi\Attribute;
 use JsonApiPhp\JsonApi\DataDocument;
+use JsonApiPhp\JsonApi\EmptyRelationship;
 use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\Meta;
@@ -14,10 +17,8 @@
 use JsonApiPhp\JsonApi\ToNull;
 use JsonApiPhp\JsonApi\ToOne;
 
-class ResourceObjectTest extends BaseTestCase
-{
-    public function testFullFledgedResourceObject()
-    {
+class ResourceObjectTest extends BaseTestCase {
+    public function testFullFledgedResourceObject() {
         $this->assertEncodesTo(
             '
             {
@@ -62,8 +63,7 @@ public function testFullFledgedResourceObject()
         );
     }
 
-    public function testRelationshipWithSingleIdLinkage()
-    {
+    public function testRelationshipWithSingleIdLinkage() {
         $this->assertEncodesTo(
             '
             {
@@ -88,8 +88,7 @@ public function testRelationshipWithSingleIdLinkage()
         );
     }
 
-    public function testRelationshipWithMultiIdLinkage()
-    {
+    public function testRelationshipWithMultiIdLinkage() {
         $this->assertEncodesTo(
             '
             {
@@ -126,8 +125,7 @@ public function testRelationshipWithMultiIdLinkage()
         );
     }
 
-    public function testRelationshipWithEmptyMultiIdLinkage()
-    {
+    public function testRelationshipWithEmptyMultiIdLinkage() {
         $this->assertEncodesTo(
             '
             {
@@ -152,29 +150,81 @@ public function testRelationshipWithEmptyMultiIdLinkage()
         );
     }
 
-    public function testCanNotCreateIdAttribute()
-    {
+    public function testRelationshipWithNoData() {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "id": "1",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new ResourceObject(
+                    'basket',
+                    '1',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'))
+                )
+            )
+        );
+
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "id": "1",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo",
+                                "self": "/bar"
+                            },
+                            "meta": {
+                                "foo": "bar"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new ResourceObject(
+                    'basket',
+                    '1',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'), new SelfLink('/bar'), new Meta('foo', 'bar'))
+                )
+            )
+        );
+    }
+
+    public function testCanNotCreateIdAttribute() {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage("Can not use 'id' as a resource field");
         new Attribute('id', 'foo');
     }
 
-    public function testCanNotCreateTypeAttribute()
-    {
+    public function testCanNotCreateTypeAttribute() {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage("Can not use 'type' as a resource field");
         new Attribute('type', 'foo');
     }
 
-    public function testCanNotCreateIdRelationship()
-    {
+    public function testCanNotCreateIdRelationship() {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage("Can not use 'id' as a resource field");
         new ToOne('id', new ResourceIdentifier('apples', '1'));
     }
 
-    public function testCanNotCreateTypeRelationship()
-    {
+    public function testCanNotCreateTypeRelationship() {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage("Can not use 'type' as a resource field");
         new ToOne('type', new ResourceIdentifier('apples', '1'));
@@ -184,8 +234,7 @@ public function testCanNotCreateTypeRelationship()
      * @dataProvider invalidCharacters
      * @param string $invalid_char
      */
-    public function testAttributeMustOnlyHaveAllowedCharacters(string $invalid_char)
-    {
+    public function testAttributeMustOnlyHaveAllowedCharacters(string $invalid_char) {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage('Invalid character in a member name');
         new Attribute("foo{$invalid_char}bar", 'plus can not be used');
@@ -195,15 +244,13 @@ public function testAttributeMustOnlyHaveAllowedCharacters(string $invalid_char)
      * @dataProvider invalidCharacters
      * @param string $invalid_char
      */
-    public function testRelationshipMustOnlyHaveAllowedCharacters(string $invalid_char)
-    {
+    public function testRelationshipMustOnlyHaveAllowedCharacters(string $invalid_char) {
         $this->expectException(\DomainException::class);
         $this->expectExceptionMessage('Invalid character in a member name');
         new ToNull("foo{$invalid_char}bar");
     }
 
-    public function invalidCharacters()
-    {
+    public function invalidCharacters() {
         return [
             ['+'],
             ['!'],
@@ -213,8 +260,7 @@ public function invalidCharacters()
         ];
     }
 
-    public function testResourceFieldsMustBeUnique()
-    {
+    public function testResourceFieldsMustBeUnique() {
         $this->expectException(\LogicException::class);
         $this->expectExceptionMessage("Field 'foo' already exists");
         new ResourceObject(
@@ -225,8 +271,7 @@ public function testResourceFieldsMustBeUnique()
         );
     }
 
-    public function testNameValidation()
-    {
+    public function testNameValidation() {
         $this->expectException(\DomainException::class);
         new ResourceObject('invalid:id', 'foo');
     }
diff --git a/test/benchmarks/compound10k.php b/test/benchmarks/compound10k.php
index c1a0ae2..44664fd 100644
--- a/test/benchmarks/compound10k.php
+++ b/test/benchmarks/compound10k.php
@@ -1,4 +1,6 @@
-<?php declare(strict_types=1);
+<?php
+
+declare(strict_types=1);
 
 use JsonApiPhp\JsonApi\Attribute;
 use JsonApiPhp\JsonApi\CompoundDocument;
@@ -48,8 +50,8 @@
         new Attribute('body', 'First!'),
         new SelfLink('http://example.com/comments/5'),
         new ToOne('author', new ResourceIdentifier('people', '2'))
-
     );
+
     $comment12 = new ResourceObject(
         'comments',
         '12',