diff --git a/examples/compound_doc.php b/examples/compound_doc.php
index 61842ef..ddb6fd9 100644
--- a/examples/compound_doc.php
+++ b/examples/compound_doc.php
@@ -7,8 +7,9 @@
 use JsonApiPhp\JsonApi\Link\NextLink;
 use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
-use JsonApiPhp\JsonApi\PaginatedResourceCollection;
+use JsonApiPhp\JsonApi\PaginatedCollection;
 use JsonApiPhp\JsonApi\Pagination;
+use JsonApiPhp\JsonApi\ResourceCollection;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
 use JsonApiPhp\JsonApi\ResourceIdentifierCollection;
 use JsonApiPhp\JsonApi\ResourceObject;
@@ -39,34 +40,36 @@
     '12',
     new Attribute('body', 'I like XML better'),
     new SelfLink('http://example.com/comments/12'),
-    new ToOne('author', $dan->toIdentifier())
+    new ToOne('author', $dan->identifier())
 );
 
 $document = new CompoundDocument(
-    new PaginatedResourceCollection(
+    new PaginatedCollection(
         new Pagination(
             new NextLink('http://example.com/articles?page[offset]=2'),
             new LastLink('http://example.com/articles?page[offset]=10')
         ),
-        new ResourceObject(
-            'articles',
-            '1',
-            new Attribute('title', 'JSON API paints my bikeshed!'),
-            new SelfLink('http://example.com/articles/1'),
-            new ToOne(
-                'author',
-                $dan->toIdentifier(),
-                new SelfLink('http://example.com/articles/1/relationships/author'),
-                new RelatedLink('http://example.com/articles/1/author')
-            ),
-            new ToMany(
-                'comments',
-                new ResourceIdentifierCollection(
-                    $comment05->toIdentifier(),
-                    $comment12->toIdentifier()
+        new ResourceCollection(
+            new ResourceObject(
+                'articles',
+                '1',
+                new Attribute('title', 'JSON API paints my bikeshed!'),
+                new SelfLink('http://example.com/articles/1'),
+                new ToOne(
+                    'author',
+                    $dan->identifier(),
+                    new SelfLink('http://example.com/articles/1/relationships/author'),
+                    new RelatedLink('http://example.com/articles/1/author')
                 ),
-                new SelfLink('http://example.com/articles/1/relationships/comments'),
-                new RelatedLink('http://example.com/articles/1/comments')
+                new ToMany(
+                    'comments',
+                    new ResourceIdentifierCollection(
+                        $comment05->identifier(),
+                        $comment12->identifier()
+                    ),
+                    new SelfLink('http://example.com/articles/1/relationships/comments'),
+                    new RelatedLink('http://example.com/articles/1/comments')
+                )
             )
         )
     ),
diff --git a/src/Attribute.php b/src/Attribute.php
index fb2984f..69c6621 100644
--- a/src/Attribute.php
+++ b/src/Attribute.php
@@ -5,6 +5,9 @@
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
 
+/**
+ * @see http://jsonapi.org/format/#document-resource-object-attributes
+ */
 final class Attribute implements ResourceField
 {
     use ResourceFieldTrait;
@@ -17,7 +20,7 @@ public function __construct(string $name, $val)
         $this->val = $val;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         child($o, 'attributes')->{$this->name} = $this->val;
     }
diff --git a/src/CompoundDocument.php b/src/CompoundDocument.php
index 05bc119..7e84887 100644
--- a/src/CompoundDocument.php
+++ b/src/CompoundDocument.php
@@ -5,6 +5,10 @@
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
+/**
+ * A Document with the "included" member
+ * @see http://jsonapi.org/format/#document-compound-documents
+ */
 final class CompoundDocument implements \JsonSerializable
 {
     private $doc;
diff --git a/src/DataDocument.php b/src/DataDocument.php
index 68fa2cd..d8b986d 100644
--- a/src/DataDocument.php
+++ b/src/DataDocument.php
@@ -5,6 +5,10 @@
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
+/**
+ * A Document containing the "data" member
+ * @see http://jsonapi.org/format/#document-top-level
+ */
 final class DataDocument implements \JsonSerializable
 {
     private $value;
diff --git a/src/Error.php b/src/Error.php
index 65b3071..65a012d 100644
--- a/src/Error.php
+++ b/src/Error.php
@@ -5,6 +5,10 @@
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
+/**
+ * An Error Object
+ * @see
+ */
 final class Error implements ErrorDocumentMember
 {
     private $error;
@@ -17,7 +21,7 @@ public function __construct(ErrorMember ...$members)
         }
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->errors[] = $this->error;
     }
diff --git a/src/ErrorDocument.php b/src/ErrorDocument.php
index 382d5a6..e4a52e5 100644
--- a/src/ErrorDocument.php
+++ b/src/ErrorDocument.php
@@ -4,21 +4,25 @@
 
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
 
+/**
+ * A Document containing an array of Error objects
+ * @see http://jsonapi.org/format/#document-top-level
+ */
 final class ErrorDocument implements \JsonSerializable
 {
-    private $doc;
+    private $obj;
 
     public function __construct(Error $error, ErrorDocumentMember ...$members)
     {
-        $this->doc = (object) [];
-        $error->attachTo($this->doc);
+        $this->obj = (object) [];
+        $error->attachTo($this->obj);
         foreach ($members as $member) {
-            $member->attachTo($this->doc);
+            $member->attachTo($this->obj);
         }
     }
 
     public function jsonSerialize()
     {
-        return $this->doc;
+        return $this->obj;
     }
 }
diff --git a/src/Included.php b/src/Included.php
index 30da0c0..04c8d1c 100644
--- a/src/Included.php
+++ b/src/Included.php
@@ -3,7 +3,6 @@
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Attachable;
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
 final class Included implements Attachable
@@ -13,34 +12,33 @@ final class Included implements Attachable
      */
     private $resources = [];
 
-    private $ids;
+    private $identifiers = [];
 
     public function __construct(ResourceObject ...$resources)
     {
-        $this->ids = new IdentifierRegistry();
         foreach ($resources as $resource) {
-            $string_id = $resource->key();
-            if (isset($this->resources[$string_id])) {
-                throw new \LogicException("Resource $string_id is already included");
+            $key = $resource->key();
+            if (isset($this->resources[$key])) {
+                throw new \LogicException("Resource $resource is already included");
             }
-            $this->resources[$string_id] = $resource;
-            $resource->registerIn($this->ids);
+            $this->resources[$key] = $resource;
+            $resource->registerIn($this->identifiers);
         }
     }
 
     public function validateLinkage(PrimaryData $data): void
     {
-        $dataRegistry = new IdentifierRegistry();
-        $data->registerIn($dataRegistry);
+        $registry = [];
+        $data->registerIn($registry);
         foreach ($this->resources as $resource) {
-            if ($dataRegistry->has($resource->key()) || $this->ids->has($resource->key())) {
+            if (isset($registry[$resource->key()]) || isset($this->identifiers[$resource->key()])) {
                 continue;
             }
-            throw new \LogicException('Full linkage required for '.$resource->key());
+            throw new \LogicException('Full linkage required for '.$resource);
         }
     }
 
-    public function attachTo(object $o)
+    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 fbbe24c..cccd352 100644
--- a/src/Internal/Attachable.php
+++ b/src/Internal/Attachable.php
@@ -7,5 +7,5 @@
  */
 interface Attachable
 {
-    public function attachTo(object $o);
+    public function attachTo(object $o): void;
 }
diff --git a/src/Internal/Collection.php b/src/Internal/Collection.php
new file mode 100644
index 0000000..e0e73d0
--- /dev/null
+++ b/src/Internal/Collection.php
@@ -0,0 +1,7 @@
+<?php declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Internal;
+
+interface Collection extends PrimaryData
+{
+}
diff --git a/src/Internal/Identifier.php b/src/Internal/Identifier.php
index 01885b9..fa435a1 100644
--- a/src/Internal/Identifier.php
+++ b/src/Internal/Identifier.php
@@ -7,5 +7,9 @@
  */
 interface Identifier
 {
-    public function registerIn(IdentifierRegistry $registry);
+    /**
+     * @internal
+     * @param array $registry
+     */
+    public function registerIn(array &$registry): void;
 }
diff --git a/src/Internal/IdentifierRegistry.php b/src/Internal/IdentifierRegistry.php
deleted file mode 100644
index e79e474..0000000
--- a/src/Internal/IdentifierRegistry.php
+++ /dev/null
@@ -1,27 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi\Internal;
-
-/**
- * @internal
- */
-final class IdentifierRegistry
-{
-    private $ids = [];
-
-    public function add(string $id): void
-    {
-        $this->ids[$id] = true;
-    }
-
-    public function has(string $id): bool
-    {
-        return isset($this->ids[$id]);
-    }
-
-    public function merge(IdentifierRegistry $registry)
-    {
-        $this->ids = array_merge($this->ids, $registry->ids);
-    }
-}
diff --git a/src/Internal/IdentityTrait.php b/src/Internal/IdentityTrait.php
deleted file mode 100644
index b46ecd4..0000000
--- a/src/Internal/IdentityTrait.php
+++ /dev/null
@@ -1,29 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi\Internal;
-
-/**
- * @internal
- */
-trait IdentityTrait
-{
-    /**
-     * @var string
-     */
-    private $id;
-
-    /**
-     * @var string
-     */
-    private $type;
-
-    /**
-     * @internal
-     * @return string
-     */
-    public function key(): string
-    {
-        return "{$this->type}:{$this->id}";
-    }
-}
diff --git a/src/Internal/LinkTrait.php b/src/Internal/LinkTrait.php
index 9d957bb..006c61a 100644
--- a/src/Internal/LinkTrait.php
+++ b/src/Internal/LinkTrait.php
@@ -1,5 +1,4 @@
-<?php
-declare(strict_types=1);
+<?php declare(strict_types=1);
 
 namespace JsonApiPhp\JsonApi\Internal;
 
diff --git a/src/Internal/ToManyMember.php b/src/Internal/ToManyMember.php
new file mode 100644
index 0000000..52dd2f7
--- /dev/null
+++ b/src/Internal/ToManyMember.php
@@ -0,0 +1,7 @@
+<?php declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Internal;
+
+interface ToManyMember extends ToOneMember
+{
+}
diff --git a/src/Internal/RelationshipMember.php b/src/Internal/ToOneMember.php
similarity index 66%
rename from src/Internal/RelationshipMember.php
rename to src/Internal/ToOneMember.php
index 092e78b..f65acd4 100644
--- a/src/Internal/RelationshipMember.php
+++ b/src/Internal/ToOneMember.php
@@ -5,6 +5,6 @@
 /**
  * @internal
  */
-interface RelationshipMember extends Attachable
+interface ToOneMember extends Attachable
 {
 }
diff --git a/src/JsonApi.php b/src/JsonApi.php
index 9293049..c0f2220 100644
--- a/src/JsonApi.php
+++ b/src/JsonApi.php
@@ -8,20 +8,20 @@
 
 final class JsonApi implements MetaDocumentMember, DataDocumentMember, ErrorDocumentMember
 {
-    private $jsonapi;
+    private $obj;
 
     public function __construct(string $version = '1.0', Meta $meta = null)
     {
-        $this->jsonapi = (object) [
+        $this->obj = (object) [
             'version' => $version,
         ];
         if ($meta) {
-            $meta->attachTo($this->jsonapi);
+            $meta->attachTo($this->obj);
         }
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
-        $o->jsonapi = $this->jsonapi;
+        $o->jsonapi = $this->obj;
     }
 }
diff --git a/src/Link/AboutLink.php b/src/Link/AboutLink.php
index 3da5849..31fe133 100644
--- a/src/Link/AboutLink.php
+++ b/src/Link/AboutLink.php
@@ -10,7 +10,7 @@ final class AboutLink implements ErrorMember
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 12b10ed..f41ab3c 100644
--- a/src/Link/FirstLink.php
+++ b/src/Link/FirstLink.php
@@ -10,7 +10,7 @@ final class FirstLink implements PaginationLink
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 b745f4a..b174cb2 100644
--- a/src/Link/LastLink.php
+++ b/src/Link/LastLink.php
@@ -10,7 +10,7 @@ final class LastLink implements PaginationLink
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 1c60fb9..e1cce27 100644
--- a/src/Link/NextLink.php
+++ b/src/Link/NextLink.php
@@ -10,7 +10,7 @@ final class NextLink implements PaginationLink
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 bbc0b6b..99a61c3 100644
--- a/src/Link/PrevLink.php
+++ b/src/Link/PrevLink.php
@@ -10,7 +10,7 @@ final class PrevLink implements PaginationLink
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 ca80378..c0005ff 100644
--- a/src/Link/RelatedLink.php
+++ b/src/Link/RelatedLink.php
@@ -3,14 +3,14 @@
 namespace JsonApiPhp\JsonApi\Link;
 
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
-use JsonApiPhp\JsonApi\Internal\RelationshipMember;
+use JsonApiPhp\JsonApi\Internal\ToOneMember;
 use function JsonApiPhp\JsonApi\child;
 
-final class RelatedLink implements RelationshipMember
+final class RelatedLink implements ToOneMember
 {
     use LinkTrait;
 
-    public function attachTo(object $o)
+    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 3817dd3..17d96cd 100644
--- a/src/Link/SelfLink.php
+++ b/src/Link/SelfLink.php
@@ -4,11 +4,11 @@
 
 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, RelationshipMember
+final class SelfLink implements DataDocumentMember, ResourceMember, ToOneMember
 {
     use LinkTrait;
 
diff --git a/src/Meta.php b/src/Meta.php
index b8252cb..e186648 100644
--- a/src/Meta.php
+++ b/src/Meta.php
@@ -6,10 +6,10 @@
 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, RelationshipMember
+final class Meta implements ErrorMember, ErrorDocumentMember, MetaDocumentMember, DataDocumentMember, ResourceMember, ToOneMember
 {
     /**
      * @var string
@@ -26,7 +26,7 @@ public function __construct(string $key, $value)
         $this->value = $value;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         child($o, 'meta')->{$this->key} = $this->value;
     }
diff --git a/src/NullData.php b/src/NullData.php
index 768102d..26dd8a5 100644
--- a/src/NullData.php
+++ b/src/NullData.php
@@ -2,17 +2,16 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
 final class NullData implements PrimaryData
 {
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->data = null;
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
     }
 }
diff --git a/src/PaginatedCollection.php b/src/PaginatedCollection.php
new file mode 100644
index 0000000..8b8e0bd
--- /dev/null
+++ b/src/PaginatedCollection.php
@@ -0,0 +1,39 @@
+<?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);
+    }
+
+    /**
+     * @internal
+     * @param array $registry
+     */
+    public function registerIn(array &$registry): void
+    {
+        $this->collection->registerIn($registry);
+    }
+}
diff --git a/src/PaginatedResourceCollection.php b/src/PaginatedResourceCollection.php
deleted file mode 100644
index cb2a633..0000000
--- a/src/PaginatedResourceCollection.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi;
-
-class PaginatedResourceCollection extends ResourceCollection
-{
-    /**
-     * @var Pagination
-     */
-    private $pagination;
-
-    public function __construct(Pagination $pagination, ResourceObject ...$resources)
-    {
-        parent::__construct(...$resources);
-        $this->pagination = $pagination;
-    }
-
-    public function attachTo(object $o): void
-    {
-        parent::attachTo($o);
-        $this->pagination->attachTo($o);
-    }
-}
diff --git a/src/PaginatedResourceIdentifierCollection.php b/src/PaginatedResourceIdentifierCollection.php
deleted file mode 100644
index ea4e432..0000000
--- a/src/PaginatedResourceIdentifierCollection.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi;
-
-class PaginatedResourceIdentifierCollection extends ResourceIdentifierCollection
-{
-    /**
-     * @var Pagination
-     */
-    private $pagination;
-
-    public function __construct(Pagination $pagination, ResourceIdentifier ...$identifiers)
-    {
-        parent::__construct(...$identifiers);
-        $this->pagination = $pagination;
-    }
-
-    public function attachTo(object $o)
-    {
-        parent::attachTo($o);
-        $this->pagination->attachTo($o);
-    }
-}
diff --git a/src/Pagination.php b/src/Pagination.php
index ba8ab41..aa4f9c3 100644
--- a/src/Pagination.php
+++ b/src/Pagination.php
@@ -3,10 +3,10 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\Attachable;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
+use JsonApiPhp\JsonApi\Internal\ToManyMember;
 
-class Pagination implements Attachable
+class Pagination implements ToManyMember
 {
     /**
      * @var PaginationLink[]
@@ -18,7 +18,7 @@ public function __construct(PaginationLink ...$links)
         $this->links = $links;
     }
 
-    public function attachTo(object $o)
+    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 e29ee55..77ac09f 100644
--- a/src/ResourceCollection.php
+++ b/src/ResourceCollection.php
@@ -2,10 +2,10 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
+use JsonApiPhp\JsonApi\Internal\Collection;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-class ResourceCollection implements PrimaryData
+class ResourceCollection implements PrimaryData, Collection
 {
     /**
      * @var ResourceObject[]
@@ -17,7 +17,7 @@ public function __construct(ResourceObject ...$resources)
         $this->resources = $resources;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->data = [];
         foreach ($this->resources as $resource) {
@@ -25,7 +25,7 @@ public function attachTo(object $o)
         }
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
         foreach ($this->resources as $resource) {
             $resource->registerIn($registry);
diff --git a/src/ResourceIdentifier.php b/src/ResourceIdentifier.php
index 7d170b0..221e096 100644
--- a/src/ResourceIdentifier.php
+++ b/src/ResourceIdentifier.php
@@ -2,14 +2,19 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
-use JsonApiPhp\JsonApi\Internal\IdentityTrait;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
 final class ResourceIdentifier implements PrimaryData
 {
-    use IdentityTrait;
     private $obj;
+    /**
+     * @var string
+     */
+    private $type;
+    /**
+     * @var string
+     */
+    private $id;
 
     public function __construct(string $type, string $id, Meta $meta = null)
     {
@@ -28,7 +33,7 @@ public function __construct(string $type, string $id, Meta $meta = null)
         $this->id = $id;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->data = $this->obj;
     }
@@ -38,8 +43,8 @@ public function attachToCollection(object $o): void
         $o->data[] = $this->obj;
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
-        $registry->add($this->key());
+        $registry[compositeKey($this->type, $this->id)] = true;
     }
 }
diff --git a/src/ResourceIdentifierCollection.php b/src/ResourceIdentifierCollection.php
index 1786c1d..d13eb11 100644
--- a/src/ResourceIdentifierCollection.php
+++ b/src/ResourceIdentifierCollection.php
@@ -2,10 +2,10 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
+use JsonApiPhp\JsonApi\Internal\Collection;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 
-class ResourceIdentifierCollection implements PrimaryData
+class ResourceIdentifierCollection implements PrimaryData, Collection
 {
     /**
      * @var ResourceIdentifier[]
@@ -17,7 +17,7 @@ public function __construct(ResourceIdentifier ...$identifiers)
         $this->identifiers = $identifiers;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->data = [];
         foreach ($this->identifiers as $identifier) {
@@ -25,7 +25,7 @@ public function attachTo(object $o)
         }
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
         foreach ($this->identifiers as $identifier) {
             $identifier->registerIn($registry);
diff --git a/src/ResourceObject.php b/src/ResourceObject.php
index 02701d2..de0a478 100644
--- a/src/ResourceObject.php
+++ b/src/ResourceObject.php
@@ -3,26 +3,28 @@
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Identifier;
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
-use JsonApiPhp\JsonApi\Internal\IdentityTrait;
 use JsonApiPhp\JsonApi\Internal\PrimaryData;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
 
 final class ResourceObject implements PrimaryData
 {
-    use IdentityTrait;
     private $obj;
-    private $registry;
+    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->type = $type;
-        $this->id = $id;
-        $this->registry = new IdentifierRegistry();
         $this->obj = (object) ['type' => $type, 'id' => $id];
         $fields = [];
         foreach ($members as $member) {
@@ -38,19 +40,26 @@ public function __construct(string $type, string $id, ResourceMember ...$members
             }
             $member->attachTo($this->obj);
         }
+        $this->type = $type;
+        $this->id = $id;
     }
 
-    public function toIdentifier(): ResourceIdentifier
+    public function identifier(): ResourceIdentifier
     {
         return new ResourceIdentifier($this->type, $this->id);
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function key(): string
+    {
+        return compositeKey($this->type, $this->id);
+    }
+
+    public function registerIn(array &$registry): void
     {
-        $registry->merge($this->registry);
+        $registry = array_merge($registry, $this->registry);
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $o->data = $this->obj;
     }
@@ -64,4 +73,9 @@ public function attachToCollection(object $o): void
     {
         $o->data[] = $this->obj;
     }
+
+    public function __toString(): string
+    {
+        return $this->key();
+    }
 }
diff --git a/src/ToMany.php b/src/ToMany.php
index d8b2ba3..94b873b 100644
--- a/src/ToMany.php
+++ b/src/ToMany.php
@@ -3,16 +3,15 @@
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Identifier;
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
-use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
+use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
 final class ToMany implements Identifier, ResourceField
 {
     use ResourceFieldTrait;
     /**
-     * @var RelationshipMember[]
+     * @var ToOneMember[]
      */
     private $members;
     /**
@@ -20,7 +19,7 @@ final class ToMany implements Identifier, ResourceField
      */
     private $collection;
 
-    public function __construct(string $name, ResourceIdentifierCollection $collection, RelationshipMember ...$members)
+    public function __construct(string $name, ResourceIdentifierCollection $collection, ToOneMember ...$members)
     {
         $this->validateFieldName($name);
         $this->name = $name;
@@ -28,7 +27,7 @@ public function __construct(string $name, ResourceIdentifierCollection $collecti
         $this->collection = $collection;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $rel = child(child($o, 'relationships'), $this->name);
         $rel->data = [];
@@ -38,7 +37,7 @@ public function attachTo(object $o)
         }
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
         $this->collection->registerIn($registry);
     }
diff --git a/src/ToNull.php b/src/ToNull.php
index c163d67..0efaef0 100644
--- a/src/ToNull.php
+++ b/src/ToNull.php
@@ -2,26 +2,26 @@
 
 namespace JsonApiPhp\JsonApi;
 
-use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
+use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
 final class ToNull implements ResourceField
 {
     use ResourceFieldTrait;
     /**
-     * @var RelationshipMember[]
+     * @var ToOneMember[]
      */
     private $members;
 
-    public function __construct(string $name, RelationshipMember ...$members)
+    public function __construct(string $name, ToOneMember ...$members)
     {
         $this->validateFieldName($name);
         $this->name = $name;
         $this->members = $members;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         $obj = combine(...$this->members);
         $obj->data = null;
diff --git a/src/ToOne.php b/src/ToOne.php
index b696710..c6d32da 100644
--- a/src/ToOne.php
+++ b/src/ToOne.php
@@ -3,10 +3,9 @@
 namespace JsonApiPhp\JsonApi;
 
 use JsonApiPhp\JsonApi\Internal\Identifier;
-use JsonApiPhp\JsonApi\Internal\IdentifierRegistry;
-use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
+use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
 final class ToOne implements Identifier, ResourceField
 {
@@ -19,7 +18,7 @@ final class ToOne implements Identifier, ResourceField
 
     private $obj;
 
-    public function __construct(string $name, ResourceIdentifier $identifier, RelationshipMember ...$members)
+    public function __construct(string $name, ResourceIdentifier $identifier, ToOneMember ...$members)
     {
         $this->validateFieldName($name);
         $this->name = $name;
@@ -27,12 +26,12 @@ public function __construct(string $name, ResourceIdentifier $identifier, Relati
         $this->identifier = $identifier;
     }
 
-    public function attachTo(object $o)
+    public function attachTo(object $o): void
     {
         child($o, 'relationships')->{$this->name} = $this->obj;
     }
 
-    public function registerIn(IdentifierRegistry $registry)
+    public function registerIn(array &$registry): void
     {
         $this->identifier->registerIn($registry);
     }
diff --git a/src/functions.php b/src/functions.php
index 458cb53..d7c89a4 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -25,3 +25,8 @@ function isValidName(string $name): bool
 {
     return preg_match('/^(?=[^-_ ])[a-zA-Z0-9\x{0080}-\x{FFFF}-_ ]*(?<=[^-_ ])$/u', $name) === 1;
 }
+
+function compositeKey(string $type, string $id): string
+{
+    return "{$type}:{$id}";
+}
diff --git a/test/CompoundDocumentTest.php b/test/CompoundDocumentTest.php
index 01ede9c..3d62ff9 100644
--- a/test/CompoundDocumentTest.php
+++ b/test/CompoundDocumentTest.php
@@ -10,7 +10,7 @@
 use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\NullData;
-use JsonApiPhp\JsonApi\PaginatedResourceCollection;
+use JsonApiPhp\JsonApi\PaginatedCollection;
 use JsonApiPhp\JsonApi\Pagination;
 use JsonApiPhp\JsonApi\ResourceCollection;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
@@ -45,34 +45,36 @@ public function testOfficialDocsExample()
             '12',
             new Attribute('body', 'I like XML better'),
             new SelfLink('http://example.com/comments/12'),
-            new ToOne('author', $dan->toIdentifier())
+            new ToOne('author', $dan->identifier())
         );
 
         $document = new CompoundDocument(
-            new PaginatedResourceCollection(
+            new PaginatedCollection(
                 new Pagination(
                     new NextLink('http://example.com/articles?page[offset]=2'),
                     new LastLink('http://example.com/articles?page[offset]=10')
                 ),
-                new ResourceObject(
-                    'articles',
-                    '1',
-                    new Attribute('title', 'JSON API paints my bikeshed!'),
-                    new SelfLink('http://example.com/articles/1'),
-                    new ToOne(
-                        'author',
-                        $dan->toIdentifier(),
-                        new SelfLink('http://example.com/articles/1/relationships/author'),
-                        new RelatedLink('http://example.com/articles/1/author')
-                    ),
-                    new ToMany(
-                        'comments',
-                        new ResourceIdentifierCollection(
-                            $comment05->toIdentifier(),
-                            $comment12->toIdentifier()
+                new ResourceCollection(
+                    new ResourceObject(
+                        'articles',
+                        '1',
+                        new Attribute('title', 'JSON API paints my bikeshed!'),
+                        new SelfLink('http://example.com/articles/1'),
+                        new ToOne(
+                            'author',
+                            $dan->identifier(),
+                            new SelfLink('http://example.com/articles/1/relationships/author'),
+                            new RelatedLink('http://example.com/articles/1/author')
                         ),
-                        new SelfLink('http://example.com/articles/1/relationships/comments'),
-                        new RelatedLink('http://example.com/articles/1/comments')
+                        new ToMany(
+                            'comments',
+                            new ResourceIdentifierCollection(
+                                $comment05->identifier(),
+                                $comment12->identifier()
+                            ),
+                            new SelfLink('http://example.com/articles/1/relationships/comments'),
+                            new RelatedLink('http://example.com/articles/1/comments')
+                        )
                     )
                 )
             ),
@@ -225,7 +227,7 @@ public function testIncludedResourceMayBeIdentifiedByLinkageInPrimaryData()
         $article = new ResourceObject(
             'articles',
             '1',
-            new ToOne('author', $author->toIdentifier())
+            new ToOne('author', $author->identifier())
         );
         $doc = new CompoundDocument($article, new Included($author));
         $this->assertNotEmpty($doc);
@@ -238,12 +240,12 @@ public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
             'books',
             '2',
             new Attribute('name', 'Domain Driven Design'),
-            new ToOne('author', $writer->toIdentifier())
+            new ToOne('author', $writer->identifier())
         );
         $cart = new ResourceObject(
             'shopping-carts',
             '1',
-            new ToMany('contents', new ResourceIdentifierCollection($book->toIdentifier()))
+            new ToMany('contents', new ResourceIdentifierCollection($book->identifier()))
         );
         $doc = new CompoundDocument($cart, new Included($book, $writer));
         $this->assertNotEmpty($doc);
@@ -257,6 +259,6 @@ public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
     public function testCanNotBeManyIncludedResourcesWithEqualIdentifiers()
     {
         $apple = new ResourceObject('apples', '1');
-        new CompoundDocument($apple->toIdentifier(), new Included($apple, $apple));
+        new CompoundDocument($apple->identifier(), new Included($apple, $apple));
     }
 }
diff --git a/test/PaginationTest.php b/test/PaginationTest.php
index e00ab53..17ee668 100644
--- a/test/PaginationTest.php
+++ b/test/PaginationTest.php
@@ -10,10 +10,11 @@
 use JsonApiPhp\JsonApi\Link\LastLink;
 use JsonApiPhp\JsonApi\Link\NextLink;
 use JsonApiPhp\JsonApi\Link\PrevLink;
-use JsonApiPhp\JsonApi\PaginatedResourceCollection;
-use JsonApiPhp\JsonApi\PaginatedResourceIdentifierCollection;
+use JsonApiPhp\JsonApi\PaginatedCollection;
 use JsonApiPhp\JsonApi\Pagination;
+use JsonApiPhp\JsonApi\ResourceCollection;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
+use JsonApiPhp\JsonApi\ResourceIdentifierCollection;
 use JsonApiPhp\JsonApi\ResourceObject;
 use JsonApiPhp\JsonApi\ToMany;
 
@@ -37,15 +38,17 @@ public function testPaginatedResourceCollection()
             }
             ',
             new DataDocument(
-                new PaginatedResourceCollection(
+                new PaginatedCollection(
                     new Pagination(
                         new FirstLink('http://example.com/fruits?page=first'),
                         new PrevLink('http://example.com/fruits?page=3'),
                         new NextLink('http://example.com/fruits?page=5'),
                         new LastLink('http://example.com/fruits?page=last')
                     ),
-                    new ResourceObject('apples', '1'),
-                    new ResourceObject('apples', '2')
+                    new ResourceCollection(
+                        new ResourceObject('apples', '1'),
+                        new ResourceObject('apples', '2')
+                    )
                 )
             )
         );
@@ -86,15 +89,15 @@ public function testPaginatedResourceIdentifierCollection()
                     '1',
                     new ToMany(
                         'fruits',
-                        new PaginatedResourceIdentifierCollection(
-                            new Pagination(
-                                new FirstLink('http://example.com/basket/1/fruits?page=first'),
-                                new PrevLink('http://example.com/basket/1/fruits?page=3'),
-                                new NextLink('http://example.com/basket/1/fruits?page=5'),
-                                new LastLink('http://example.com/basket/1/fruits?page=last')
-                            ),
+                        new ResourceIdentifierCollection(
                             new ResourceIdentifier('apples', '1'),
                             new ResourceIdentifier('apples', '2')
+                        ),
+                        new Pagination(
+                            new FirstLink('http://example.com/basket/1/fruits?page=first'),
+                            new PrevLink('http://example.com/basket/1/fruits?page=3'),
+                            new NextLink('http://example.com/basket/1/fruits?page=5'),
+                            new LastLink('http://example.com/basket/1/fruits?page=last')
                         )
                     )
                 ),
diff --git a/test/benchmarks/compound10k.php b/test/benchmarks/compound10k.php
index 8f2fad4..c1a0ae2 100644
--- a/test/benchmarks/compound10k.php
+++ b/test/benchmarks/compound10k.php
@@ -19,8 +19,9 @@
 use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\Meta;
-use JsonApiPhp\JsonApi\PaginatedResourceCollection;
+use JsonApiPhp\JsonApi\PaginatedCollection;
 use JsonApiPhp\JsonApi\Pagination;
+use JsonApiPhp\JsonApi\ResourceCollection;
 use JsonApiPhp\JsonApi\ResourceIdentifier;
 use JsonApiPhp\JsonApi\ResourceIdentifierCollection;
 use JsonApiPhp\JsonApi\ResourceObject;
@@ -54,34 +55,36 @@
         '12',
         new Attribute('body', 'I like XML better'),
         new SelfLink('http://example.com/comments/12'),
-        new ToOne('author', $dan->toIdentifier())
+        new ToOne('author', $dan->identifier())
     );
 
-    $data_document = new CompoundDocument(
-        new PaginatedResourceCollection(
+    $document = new CompoundDocument(
+        new PaginatedCollection(
             new Pagination(
                 new NextLink('http://example.com/articles?page[offset]=2'),
                 new LastLink('http://example.com/articles?page[offset]=10')
             ),
-            new ResourceObject(
-                'articles',
-                '1',
-                new Attribute('title', 'JSON API paints my bikeshed!'),
-                new SelfLink('http://example.com/articles/1'),
-                new ToOne(
-                    'author',
-                    $dan->toIdentifier(),
-                    new SelfLink('http://example.com/articles/1/relationships/author'),
-                    new RelatedLink('http://example.com/articles/1/author')
-                ),
-                new ToMany(
-                    'comments',
-                    new ResourceIdentifierCollection(
-                        $comment05->toIdentifier(),
-                        $comment12->toIdentifier()
+            new ResourceCollection(
+                new ResourceObject(
+                    'articles',
+                    '1',
+                    new Attribute('title', 'JSON API paints my bikeshed!'),
+                    new SelfLink('http://example.com/articles/1'),
+                    new ToOne(
+                        'author',
+                        $dan->identifier(),
+                        new SelfLink('http://example.com/articles/1/relationships/author'),
+                        new RelatedLink('http://example.com/articles/1/author')
                     ),
-                    new SelfLink('http://example.com/articles/1/relationships/comments'),
-                    new RelatedLink('http://example.com/articles/1/comments')
+                    new ToMany(
+                        'comments',
+                        new ResourceIdentifierCollection(
+                            $comment05->identifier(),
+                            $comment12->identifier()
+                        ),
+                        new SelfLink('http://example.com/articles/1/relationships/comments'),
+                        new RelatedLink('http://example.com/articles/1/comments')
+                    )
                 )
             )
         ),
@@ -89,11 +92,11 @@
         new SelfLink('http://example.com/articles')
     );
 
-    $data_doc_json = json_encode($data_document);
+    $data_doc_json = json_encode($document);
 }
 
 for ($count = 0; $count < 1000; $count++) {
-    $error_doc = new ErrorDocument(
+    $error = new ErrorDocument(
         new Error(
             new Id('1'),
             new AboutLink('/errors/not_found'),
@@ -109,7 +112,7 @@
         new JsonApi()
     );
 
-    $error_doc_json = json_encode($error_doc);
+    $error_doc_json = json_encode($error);
 }
 
 echo $data_doc_json;