diff --git a/src/Document.php b/src/Document.php
index fdec9ce..a102132 100644
--- a/src/Document.php
+++ b/src/Document.php
@@ -15,7 +15,13 @@
 use JsonApiPhp\JsonApi\Document\LinksTrait;
 use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\MetaTrait;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceInterface;
+use JsonApiPhp\JsonApi\Document\PrimaryData\MultiIdentifierData;
+use JsonApiPhp\JsonApi\Document\PrimaryData\MultiResourceData;
+use JsonApiPhp\JsonApi\Document\PrimaryData\NullData;
+use JsonApiPhp\JsonApi\Document\PrimaryData\PrimaryDataInterface;
+use JsonApiPhp\JsonApi\Document\PrimaryData\SingleIdentifierData;
+use JsonApiPhp\JsonApi\Document\PrimaryData\SingleResourceData;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 
 class Document implements \JsonSerializable
@@ -26,11 +32,23 @@ class Document implements \JsonSerializable
     use LinksTrait;
     use MetaTrait;
 
+    /**
+     * @var PrimaryDataInterface
+     */
     private $data;
+
+    /**
+     * @var Error[]
+     */
     private $errors;
+
     private $api;
+
+    /**
+     * @var ResourceObject[]
+     */
     private $included;
-    private $is_sparse = false;
+    private $sparse = false;
 
     private function __construct()
     {
@@ -50,17 +68,38 @@ public static function fromErrors(Error ...$errors): self
         return $doc;
     }
 
-    public static function fromResource(ResourceInterface $data): self
+    public static function fromResource(ResourceObject $resource): self
+    {
+        $doc = new self;
+        $doc->data = new SingleResourceData($resource);
+        return $doc;
+    }
+
+    public static function fromResources(ResourceObject ...$resources): self
+    {
+        $doc = new self;
+        $doc->data = new MultiResourceData(...$resources);
+        return $doc;
+    }
+
+    public static function fromIdentifier(ResourceIdentifier $identifier)
+    {
+        $doc = new self;
+        $doc->data = new SingleIdentifierData($identifier);
+        return $doc;
+    }
+
+    public static function fromIdentifiers(ResourceIdentifier... $identifiers)
     {
         $doc = new self;
-        $doc->data = $data;
+        $doc->data = new MultiIdentifierData(...$identifiers);
         return $doc;
     }
 
-    public static function fromResources(ResourceInterface ...$data): self
+    public static function nullDocument()
     {
         $doc = new self;
-        $doc->data = $data;
+        $doc->data = new NullData();
         return $doc;
     }
 
@@ -69,19 +108,27 @@ public function setApiVersion(string $version = self::DEFAULT_API_VERSION)
         $this->api['version'] = $version;
     }
 
-    public function setApiMeta(array $meta)
+    public function setApiMeta(Meta $meta)
     {
         $this->api['meta'] = $meta;
     }
 
-    public function setIncluded(ResourceObject ...$included)
+    public function setIncluded(ResourceObject ...$resources)
     {
-        $this->included = $included;
+        if (null === $this->data) {
+            throw new \LogicException('Document with no data cannot contain included resources');
+        }
+        foreach ($resources as $resource) {
+            if (isset($this->included[(string) $resource->toIdentifier()])) {
+                throw new \LogicException("Resource {$resource->toIdentifier()} is already included");
+            }
+            $this->included[(string) $resource->toIdentifier()] = $resource;
+        }
     }
 
     public function markSparse()
     {
-        $this->is_sparse = true;
+        $this->sparse = true;
     }
 
     public function jsonSerialize()
@@ -94,7 +141,7 @@ public function jsonSerialize()
                 'meta' => $this->meta,
                 'jsonapi' => $this->api,
                 'links' => $this->links,
-                'included' => $this->included,
+                'included' => $this->included ? array_values($this->included) : null,
             ],
             function ($v) {
                 return null !== $v;
@@ -104,47 +151,19 @@ function ($v) {
 
     private function enforceFullLinkage()
     {
-        if ($this->is_sparse || empty($this->included)) {
+        if ($this->sparse || empty($this->included)) {
             return;
         }
-        foreach ($this->included as $included_resource) {
-            if ($this->hasLinkTo($included_resource) || $this->anotherIncludedResourceIdentifies($included_resource)) {
+        foreach ($this->included as $included) {
+            if ($this->data->hasLinkTo($included)) {
                 continue;
             }
-            throw new \LogicException("Full linkage is required for $included_resource");
-        }
-    }
-
-    private function anotherIncludedResourceIdentifies(ResourceObject $resource): bool
-    {
-        /** @var ResourceObject $included_resource */
-        foreach ($this->included as $included_resource) {
-            if ($included_resource !== $resource && $included_resource->identifies($resource)) {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    private function hasLinkTo(ResourceObject $resource): bool
-    {
-        /** @var ResourceInterface $my_resource */
-        foreach ($this->toResources() as $my_resource) {
-            if ($my_resource->identifies($resource)) {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    private function toResources(): \Iterator
-    {
-        if ($this->data instanceof ResourceInterface) {
-            yield $this->data;
-        } elseif (is_array($this->data)) {
-            foreach ($this->data as $datum) {
-                yield $datum;
+            foreach ($this->included as $anotherIncluded) {
+                if ($anotherIncluded->identifies($included)) {
+                    continue 2;
+                }
             }
+            throw new \LogicException("Full linkage is required for {$included->toIdentifier()}");
         }
     }
 }
diff --git a/src/Document/PrimaryData/MultiIdentifierData.php b/src/Document/PrimaryData/MultiIdentifierData.php
new file mode 100644
index 0000000..93d7c1a
--- /dev/null
+++ b/src/Document/PrimaryData/MultiIdentifierData.php
@@ -0,0 +1,40 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class MultiIdentifierData implements PrimaryDataInterface
+{
+    private $identifiers;
+
+    public function __construct(ResourceIdentifier ...$identifiers)
+    {
+        $this->identifiers = $identifiers;
+    }
+
+    public function hasLinkTo(ResourceObject $resource): bool
+    {
+        foreach ($this->identifiers as $identifier) {
+            if ($identifier->identifies($resource)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->identifiers;
+    }
+}
diff --git a/src/Document/PrimaryData/MultiResourceData.php b/src/Document/PrimaryData/MultiResourceData.php
new file mode 100644
index 0000000..0e63fd8
--- /dev/null
+++ b/src/Document/PrimaryData/MultiResourceData.php
@@ -0,0 +1,39 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class MultiResourceData implements PrimaryDataInterface
+{
+    private $resources;
+
+    public function __construct(ResourceObject ...$resources)
+    {
+        $this->resources = $resources;
+    }
+
+    public function hasLinkTo(ResourceObject $resource): bool
+    {
+        foreach ($this->resources as $myResource) {
+            if ($myResource->identifies($resource)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->resources;
+    }
+}
diff --git a/src/Document/PrimaryData/NullData.php b/src/Document/PrimaryData/NullData.php
new file mode 100644
index 0000000..68882da
--- /dev/null
+++ b/src/Document/PrimaryData/NullData.php
@@ -0,0 +1,27 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class NullData implements PrimaryDataInterface
+{
+    public function hasLinkTo(ResourceObject $resource): bool
+    {
+        return false;
+    }
+
+    public function jsonSerialize()
+    {
+        return null;
+    }
+}
diff --git a/src/Document/PrimaryData/PrimaryDataInterface.php b/src/Document/PrimaryData/PrimaryDataInterface.php
new file mode 100644
index 0000000..89967c5
--- /dev/null
+++ b/src/Document/PrimaryData/PrimaryDataInterface.php
@@ -0,0 +1,21 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+interface PrimaryDataInterface extends \JsonSerializable
+{
+    public function hasLinkTo(ResourceObject $resource): bool;
+
+    public function jsonSerialize();
+}
diff --git a/src/Document/PrimaryData/SingleIdentifierData.php b/src/Document/PrimaryData/SingleIdentifierData.php
new file mode 100644
index 0000000..9f8959b
--- /dev/null
+++ b/src/Document/PrimaryData/SingleIdentifierData.php
@@ -0,0 +1,35 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class SingleIdentifierData implements PrimaryDataInterface
+{
+    private $identifier;
+
+    public function __construct(ResourceIdentifier $identifier)
+    {
+        $this->identifier = $identifier;
+    }
+
+    public function hasLinkTo(ResourceObject $resource): bool
+    {
+        return $this->identifier->identifies($resource);
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->identifier;
+    }
+}
diff --git a/src/Document/PrimaryData/SingleResourceData.php b/src/Document/PrimaryData/SingleResourceData.php
new file mode 100644
index 0000000..1ff8446
--- /dev/null
+++ b/src/Document/PrimaryData/SingleResourceData.php
@@ -0,0 +1,34 @@
+<?php
+/**
+ * This file is part of JSON:API implementation for PHP.
+ *
+ * (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\PrimaryData;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class SingleResourceData implements PrimaryDataInterface
+{
+    private $resource;
+
+    public function __construct(ResourceObject $resource)
+    {
+        $this->resource = $resource;
+    }
+
+    public function hasLinkTo(ResourceObject $resource): bool
+    {
+        return $this->resource->identifies($resource);
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->resource;
+    }
+}
diff --git a/src/Document/Resource/Linkage/LinkageInterface.php b/src/Document/Resource/Linkage/LinkageInterface.php
new file mode 100644
index 0000000..7340c67
--- /dev/null
+++ b/src/Document/Resource/Linkage/LinkageInterface.php
@@ -0,0 +1,20 @@
+<?php declare(strict_types=1);
+/**
+ *  This file is part of JSON:API implementation for PHP.
+ *
+ *  (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ *  For the full copyright and license information, please view the LICENSE
+ *  file that was distributed with this source code.
+ */
+
+namespace JsonApiPhp\JsonApi\Document\Resource\Linkage;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+interface LinkageInterface extends \JsonSerializable
+{
+    public function isLinkedTo(ResourceObject $resource): bool;
+
+    public function jsonSerialize();
+}
diff --git a/src/Document/Resource/Linkage/MultiLinkage.php b/src/Document/Resource/Linkage/MultiLinkage.php
new file mode 100644
index 0000000..b732335
--- /dev/null
+++ b/src/Document/Resource/Linkage/MultiLinkage.php
@@ -0,0 +1,40 @@
+<?php
+/**
+ *  This file is part of JSON:API implementation for PHP.
+ *
+ *  (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ *  For the full copyright and license information, please view the LICENSE
+ *  file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\Resource\Linkage;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class MultiLinkage implements LinkageInterface
+{
+    private $identifiers;
+
+    public function __construct(ResourceIdentifier ...$identifiers)
+    {
+        $this->identifiers = $identifiers;
+    }
+
+    public function isLinkedTo(ResourceObject $resource): bool
+    {
+        foreach ($this->identifiers as $identifier) {
+            if ($identifier->identifies($resource)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->identifiers;
+    }
+}
diff --git a/src/Document/Resource/NullResource.php b/src/Document/Resource/Linkage/NullLinkage.php
similarity index 56%
rename from src/Document/Resource/NullResource.php
rename to src/Document/Resource/Linkage/NullLinkage.php
index f30976d..e6fde43 100644
--- a/src/Document/Resource/NullResource.php
+++ b/src/Document/Resource/Linkage/NullLinkage.php
@@ -9,25 +9,19 @@
  */
 declare(strict_types=1);
 
-namespace JsonApiPhp\JsonApi\Document\Resource;
+namespace JsonApiPhp\JsonApi\Document\Resource\Linkage;
 
-final class NullResource implements ResourceInterface
-{
-    public function jsonSerialize()
-    {
-        return null;
-    }
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 
-    public function identifies(ResourceInterface $resource): bool
+final class NullLinkage implements LinkageInterface
+{
+    public function isLinkedTo(ResourceObject $resource): bool
     {
         return false;
     }
 
-    /**
-     * @deprecated to be removed in 1.0
-     */
-    public function __toString(): string
+    public function jsonSerialize()
     {
-        return 'null';
+        return null;
     }
 }
diff --git a/src/Document/Resource/Linkage/SingleLinkage.php b/src/Document/Resource/Linkage/SingleLinkage.php
new file mode 100644
index 0000000..1ed15ec
--- /dev/null
+++ b/src/Document/Resource/Linkage/SingleLinkage.php
@@ -0,0 +1,35 @@
+<?php
+/**
+ *  This file is part of JSON:API implementation for PHP.
+ *
+ *  (c) Alexey Karapetov <karapetov@gmail.com>
+ *
+ *  For the full copyright and license information, please view the LICENSE
+ *  file that was distributed with this source code.
+ */
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Document\Resource\Linkage;
+
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+
+final class SingleLinkage implements LinkageInterface
+{
+    private $identifier;
+
+    public function __construct(ResourceIdentifier $identifier)
+    {
+        $this->identifier = $identifier;
+    }
+
+    public function isLinkedTo(ResourceObject $resource): bool
+    {
+        return $this->identifier->identifies($resource);
+    }
+
+    public function jsonSerialize()
+    {
+        return $this->identifier;
+    }
+}
diff --git a/src/Document/Resource/Relationship/Linkage.php b/src/Document/Resource/Relationship/Linkage.php
deleted file mode 100644
index 54c5b76..0000000
--- a/src/Document/Resource/Relationship/Linkage.php
+++ /dev/null
@@ -1,76 +0,0 @@
-<?php
-/**
- *  This file is part of JSON:API implementation for PHP.
- *
- *  (c) Alexey Karapetov <karapetov@gmail.com>
- *
- *  For the full copyright and license information, please view the LICENSE
- *  file that was distributed with this source code.
- */
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi\Document\Resource\Relationship;
-
-use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceInterface;
-
-final class Linkage implements \JsonSerializable
-{
-    private $data;
-
-    private function __construct()
-    {
-    }
-
-    public static function nullLinkage(): self
-    {
-        return new self;
-    }
-
-    public static function emptyArrayLinkage(): self
-    {
-        $linkage = new self;
-        $linkage->data = [];
-        return $linkage;
-    }
-
-    public static function fromSingleIdentifier(ResourceIdentifier $data): self
-    {
-        $linkage = new self;
-        $linkage->data = $data;
-        return $linkage;
-    }
-
-    public static function fromManyIdentifiers(ResourceIdentifier ...$data): self
-    {
-        $linkage = new self;
-        $linkage->data = $data;
-        return $linkage;
-    }
-
-    public function isLinkedTo(ResourceInterface $resource): bool
-    {
-        foreach ($this->toLinkages() as $linkage) {
-            if ($linkage->identifies($resource)) {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    private function toLinkages(): \Generator
-    {
-        if ($this->data instanceof ResourceIdentifier) {
-            yield $this->data;
-        } elseif (is_array($this->data)) {
-            foreach ($this->data as $resource) {
-                yield $resource;
-            }
-        }
-    }
-
-    public function jsonSerialize()
-    {
-        return $this->data;
-    }
-}
diff --git a/src/Document/Resource/Relationship/Relationship.php b/src/Document/Resource/Relationship/Relationship.php
index 84b47e6..6c151db 100644
--- a/src/Document/Resource/Relationship/Relationship.php
+++ b/src/Document/Resource/Relationship/Relationship.php
@@ -14,7 +14,8 @@
 use JsonApiPhp\JsonApi\Document\LinksTrait;
 use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\MetaTrait;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceInterface;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\LinkageInterface;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 
 final class Relationship implements \JsonSerializable
 {
@@ -22,7 +23,7 @@ final class Relationship implements \JsonSerializable
     use MetaTrait;
 
     /**
-     * @var Linkage
+     * @var LinkageInterface
      */
     private $linkage = null;
 
@@ -51,14 +52,14 @@ public static function fromRelatedLink(string $link, array $meta = null): self
         return $r;
     }
 
-    public static function fromLinkage(Linkage $linkage): self
+    public static function fromLinkage(LinkageInterface $linkage): self
     {
         $r = new self;
         $r->linkage = $linkage;
         return $r;
     }
 
-    public function hasLinkageTo(ResourceInterface $resource): bool
+    public function hasLinkageTo(ResourceObject $resource): bool
     {
         return ($this->linkage && $this->linkage->isLinkedTo($resource));
     }
diff --git a/src/Document/Resource/ResourceIdentifier.php b/src/Document/Resource/ResourceIdentifier.php
index 9d28075..3b67bd8 100644
--- a/src/Document/Resource/ResourceIdentifier.php
+++ b/src/Document/Resource/ResourceIdentifier.php
@@ -12,22 +12,18 @@
 namespace JsonApiPhp\JsonApi\Document\Resource;
 
 use JsonApiPhp\JsonApi\Document\Meta;
-use JsonApiPhp\JsonApi\Document\MetaTrait;
 
-class ResourceIdentifier implements ResourceInterface
+class ResourceIdentifier implements \JsonSerializable
 {
-    use MetaTrait;
+    private $type;
+    private $id;
+    private $meta;
 
-    protected $type;
-    protected $id;
-
-    public function __construct(string $type, string $id = null, Meta $meta = null)
+    public function __construct(string $type, string $id, Meta $meta = null)
     {
         $this->type = $type;
         $this->id = $id;
-        if ($meta) {
-            $this->setMeta($meta);
-        }
+        $this->meta = $meta;
     }
 
     public function jsonSerialize()
@@ -44,19 +40,18 @@ function ($v) {
         );
     }
 
-    /**
-     * @deprecated to be removed in 1.0
-     */
+    public function identifies(ResourceObject $resource): bool
+    {
+        return $resource->toIdentifier()->equals($this);
+    }
+
     public function __toString(): string
     {
-        return sprintf("%s:%s", $this->type, $this->id ?: 'null');
+        return "$this->type:$this->id";
     }
 
-    public function identifies(ResourceInterface $resource): bool
+    private function equals(ResourceIdentifier $that)
     {
-        return $resource instanceof self
-            && $this->type === $resource->type
-            && $this->id !== null
-            && $this->id === $resource->id;
+        return $this->type === $that->type && $this->id === $that->id;
     }
 }
diff --git a/src/Document/Resource/ResourceInterface.php b/src/Document/Resource/ResourceInterface.php
deleted file mode 100644
index d8b558d..0000000
--- a/src/Document/Resource/ResourceInterface.php
+++ /dev/null
@@ -1,17 +0,0 @@
-<?php
-/**
- *  This file is part of JSON:API implementation for PHP.
- *
- *  (c) Alexey Karapetov <karapetov@gmail.com>
- *
- *  For the full copyright and license information, please view the LICENSE
- *  file that was distributed with this source code.
- */
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi\Document\Resource;
-
-interface ResourceInterface extends \JsonSerializable
-{
-    public function identifies(ResourceInterface $resource): bool;
-}
diff --git a/src/Document/Resource/ResourceObject.php b/src/Document/Resource/ResourceObject.php
index ad21ba0..9276a84 100644
--- a/src/Document/Resource/ResourceObject.php
+++ b/src/Document/Resource/ResourceObject.php
@@ -12,15 +12,34 @@
 namespace JsonApiPhp\JsonApi\Document\Resource;
 
 use JsonApiPhp\JsonApi\Document\LinksTrait;
+use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
 
-class ResourceObject extends ResourceIdentifier
+class ResourceObject implements \JsonSerializable
 {
     use LinksTrait;
 
+    private $type;
+    private $id;
+    private $meta;
     private $attributes;
+
+    /**
+     * @var Relationship[]
+     */
     private $relationships;
 
+    public function __construct(string $type, string $id = null)
+    {
+        $this->type = $type;
+        $this->id = $id;
+    }
+
+    public function setMeta(Meta $meta)
+    {
+        $this->meta = $meta;
+    }
+
     public function setAttribute(string $name, $value)
     {
         if ($this->isReservedName($name)) {
@@ -43,7 +62,7 @@ public function setRelationship(string $name, Relationship $relationship)
         $this->relationships[$name] = $relationship;
     }
 
-    public function toId(): ResourceIdentifier
+    public function toIdentifier(): ResourceIdentifier
     {
         return new ResourceIdentifier($this->type, $this->id);
     }
@@ -65,10 +84,9 @@ function ($v) {
         );
     }
 
-    public function identifies(ResourceInterface $resource): bool
+    public function identifies(ResourceObject $resource): bool
     {
         if ($this->relationships) {
-            /** @var Relationship $relationship */
             foreach ($this->relationships as $relationship) {
                 if ($relationship->hasLinkageTo($resource)) {
                     return true;
@@ -78,10 +96,6 @@ public function identifies(ResourceInterface $resource): bool
         return false;
     }
 
-    /**
-     * @param  string $name
-     * @return bool
-     */
     private function isReservedName(string $name): bool
     {
         return in_array($name, ['id', 'type']);
diff --git a/test/BaseTestCase.php b/test/BaseTestCase.php
index cc7a5a4..3c30197 100644
--- a/test/BaseTestCase.php
+++ b/test/BaseTestCase.php
@@ -17,6 +17,10 @@ abstract class BaseTestCase extends TestCase
 {
     public static function assertEncodesTo(string $expected, $obj, string $message = '')
     {
-        self::assertEquals(json_encode(json_decode($expected)), json_encode($obj), $message);
+        self::assertEquals(
+            json_decode($expected),
+            json_decode(json_encode($obj, JSON_UNESCAPED_SLASHES)),
+            $message
+        );
     }
 }
diff --git a/test/Document/CompoundDocumentTest.php b/test/Document/CompoundDocumentTest.php
index 2b3ef27..1c6794a 100644
--- a/test/Document/CompoundDocumentTest.php
+++ b/test/Document/CompoundDocumentTest.php
@@ -12,9 +12,11 @@
 namespace JsonApiPhp\JsonApi\Test\Document;
 
 use JsonApiPhp\JsonApi\Document;
-use JsonApiPhp\JsonApi\Document\Resource\NullResource;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Linkage;
+use JsonApiPhp\JsonApi\Document\Meta;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\MultiLinkage;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
@@ -41,65 +43,122 @@
 class CompoundDocumentTest extends BaseTestCase
 {
     /**
-     * In a compound document, all included resources MUST be represented as an array of resource objects
-     * in a top-level included member.
+     * Let's begin with the example from the official docs @link http://jsonapi.org/format/#document-compound-documents
      */
-    public function testIncludedResourcesRepresentedAsArray()
+    public function testOfficialDocsExample()
     {
-        $apple = new ResourceObject('apples', '1');
-        $apple->setAttribute('color', 'red');
-        $orange = new ResourceObject('oranges', '1');
-        $orange->setAttribute('color', 'orange');
-        $basket = new ResourceObject('baskets', '1');
-        $basket->setRelationship(
-            'fruits',
-            Relationship::fromLinkage(
-                Linkage::fromManyIdentifiers(
-                    $apple->toId(),
-                    $orange->toId()
-                )
-            )
+        $dan = new ResourceObject('people', '9');
+        $dan->setAttribute('first-name', 'Dan');
+        $dan->setAttribute('last-name', 'Gebhardt');
+        $dan->setAttribute('twitter', 'dgeb');
+        $dan->setLink('self', 'http://example.com/people/9');
+
+        $comment05 = new ResourceObject('comments', '5');
+        $comment05->setAttribute('body', 'First!');
+        $comment05->setLink('self', 'http://example.com/comments/5');
+        $comment05->setRelationship(
+            'author',
+            Relationship::fromLinkage(new SingleLinkage(new ResourceIdentifier('people', '2')))
         );
-        $doc = Document::fromResource($basket);
-        $doc->setIncluded($apple, $orange);
+
+        $comment12 = new ResourceObject('comments', '12');
+        $comment12->setAttribute('body', 'I like XML better');
+        $comment12->setLink('self', 'http://example.com/comments/12');
+        $comment12->setRelationship(
+            'author',
+            Relationship::fromLinkage(new SingleLinkage($dan->toIdentifier()))
+        );
+
+        $author = Relationship::fromLinkage(new SingleLinkage($dan->toIdentifier()));
+        $author->setLink('self', 'http://example.com/articles/1/relationships/author');
+        $author->setLink('related', 'http://example.com/articles/1/author');
+
+        $comments = Relationship::fromLinkage(new MultiLinkage($comment05->toIdentifier(), $comment12->toIdentifier()));
+        $comments->setLink('self', 'http://example.com/articles/1/relationships/comments');
+        $comments->setLink('related', 'http://example.com/articles/1/comments');
+
+        $article = new ResourceObject('articles', '1');
+        $article->setAttribute('title', 'JSON API paints my bikeshed!');
+        $article->setLink('self', 'http://example.com/articles/1');
+        $article->setRelationship('author', $author);
+        $article->setRelationship('comments', $comments);
+
+        $doc = Document::fromResources($article);
+        $doc->setIncluded($dan, $comment05, $comment12);
+
         $this->assertEncodesTo(
             '
             {
-                "data": {
-                    "type": "baskets",
-                    "id": "1",
-                    "relationships": {
-                        "fruits": {
-                            "data": [
-                                {
-                                    "type": "apples",
-                                    "id": "1"
-                                },
-                                {
-                                    "type": "oranges",
-                                    "id": "1"
-                                }
-                            ]
-                        }
-                    }
+              "data": [{
+                "type": "articles",
+                "id": "1",
+                "attributes": {
+                  "title": "JSON API paints my bikeshed!"
                 },
-                "included": [
-                    {
-                        "type": "apples",
-                        "id": "1",
-                        "attributes": {
-                            "color": "red"
-                        }
+                "links": {
+                  "self": "http://example.com/articles/1"
+                },
+                "relationships": {
+                  "author": {
+                    "links": {
+                      "self": "http://example.com/articles/1/relationships/author",
+                      "related": "http://example.com/articles/1/author"
                     },
-                    {
-                        "type": "oranges",
-                        "id": "1",
-                        "attributes": {
-                            "color": "orange"
-                        }
-                    }
-                ]
-            }
+                    "data": { "type": "people", "id": "9" }
+                  },
+                  "comments": {
+                    "links": {
+                      "self": "http://example.com/articles/1/relationships/comments",
+                      "related": "http://example.com/articles/1/comments"
+                    },
+                    "data": [
+                      { "type": "comments", "id": "5" },
+                      { "type": "comments", "id": "12" }
+                    ]
+                  }
+                }
+              }],
+              "included": [{
+                "type": "people",
+                "id": "9",
+                "attributes": {
+                  "first-name": "Dan",
+                  "last-name": "Gebhardt",
+                  "twitter": "dgeb"
+                },
+                "links": {
+                  "self": "http://example.com/people/9"
+                }
+              }, {
+                "type": "comments",
+                "id": "5",
+                "attributes": {
+                  "body": "First!"
+                },
+                "relationships": {
+                  "author": {
+                    "data": { "type": "people", "id": "2" }
+                  }
+                },
+                "links": {
+                  "self": "http://example.com/comments/5"
+                }
+              }, {
+                "type": "comments",
+                "id": "12",
+                "attributes": {
+                  "body": "I like XML better"
+                },
+                "relationships": {
+                  "author": {
+                    "data": { "type": "people", "id": "9" }
+                  }
+                },
+                "links": {
+                  "self": "http://example.com/comments/12"
+                }
+              }]
+            }            
             ',
             $doc
         );
@@ -111,7 +170,7 @@ public function testIncludedResourcesRepresentedAsArray()
      */
     public function testFullLinkageIsRequired()
     {
-        $doc = Document::fromResource(new NullResource);
+        $doc = Document::nullDocument();
         $doc->setIncluded(new ResourceObject('apples', '1'));
         json_encode($doc);
     }
@@ -121,7 +180,7 @@ public function testFullLinkageIsRequired()
      */
     public function testFullLinkageIsNotRequiredIfSparse()
     {
-        $doc = Document::fromResource(new NullResource);
+        $doc = Document::nullDocument();
         $doc->markSparse();
         $doc->setIncluded(new ResourceObject('apples', '1'));
         $this->assertEncodesTo(
@@ -140,34 +199,85 @@ public function testFullLinkageIsNotRequiredIfSparse()
         );
     }
 
+    /**
+     * 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.
+     */
     public function testIncludedResourceMayBeIdentifiedByPrimaryData()
     {
         $apple = new ResourceObject('apples', '1');
         $apple->setAttribute('color', 'red');
-        $doc = Document::fromResource($apple->toId());
+        $doc = Document::fromIdentifier($apple->toIdentifier());
         $doc->setIncluded($apple);
         $this->assertJson(json_encode($doc));
     }
 
-    public function testIncludedResourceMayBeIdentifiedByAnotherIncludedResource()
+    public function testIncludedResourceMayBeIdentifiedByLinkageInPrimaryData()
+    {
+        $author = new ResourceObject('people', '9');
+        $author->setAttribute('first-name', 'Dan');
+
+        $article = new ResourceObject('articles', '1');
+        $article->setAttribute('title', 'JSON API paints my bikeshed!');
+        $article->setRelationship(
+            'author',
+            Relationship::fromLinkage(new SingleLinkage($author->toIdentifier()))
+        );
+
+        $doc = Document::fromResource($article);
+        $doc->setIncluded($author);
+        $this->assertJson(json_encode($doc));
+    }
+
+    public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
+    {
+        $writer = new ResourceObject('writers', '3');
+        $writer->setAttribute('name', 'Eric Evans');
+
+        $book = new ResourceObject('books', '2');
+        $book->setAttribute('name', 'Domain Driven Design');
+        $book->setRelationship(
+            'author',
+            Relationship::fromLinkage(new SingleLinkage($writer->toIdentifier()))
+        );
+
+        $cart = new ResourceObject('shopping-carts', '1');
+        $cart->setRelationship(
+            'contents',
+            Relationship::fromLinkage(new MultiLinkage($book->toIdentifier()))
+        );
+
+        $this->assertTrue($book->identifies($writer));
+
+        $doc = Document::fromResource($cart);
+        $doc->setIncluded($book, $writer);
+        $this->assertJson(json_encode($doc));
+    }
+
+    /**
+     * 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()
     {
-        /**
-         * BasketID identifies included BasketObject
-         * BasketObject identifies included AppleObject
-         */
         $apple = new ResourceObject('apples', '1');
         $apple->setAttribute('color', 'red');
-        $basket = new ResourceObject('basket', '1');
-        $basket->setRelationship(
-            'fruits',
-            Relationship::fromLinkage(
-                Linkage::fromManyIdentifiers(
-                    $apple->toId()
-                )
-            )
-        );
-        $doc = Document::fromResource($basket->toId());
-        $doc->setIncluded($apple, $basket);
+        $doc = Document::fromIdentifier($apple->toIdentifier());
+        $doc->setIncluded($apple, $apple);
         $this->assertJson(json_encode($doc));
     }
+
+    /**
+     * If a document does not contain a top-level data key, the included member MUST NOT be present either.
+     * @expectedException \LogicException
+     * @expectedExceptionMessage Document with no data cannot contain included resources
+     */
+    public function testIncludedMustOnlyBePresentWithData()
+    {
+        $doc = Document::fromMeta(Meta::fromArray(['foo' => 'bar']));
+        $doc->setIncluded(new ResourceObject('apples', '1'));
+    }
 }
diff --git a/test/Document/DocumentTest.php b/test/Document/DocumentTest.php
index e7d5255..e1cc757 100644
--- a/test/Document/DocumentTest.php
+++ b/test/Document/DocumentTest.php
@@ -14,7 +14,6 @@
 use JsonApiPhp\JsonApi\Document;
 use JsonApiPhp\JsonApi\Document\Error;
 use JsonApiPhp\JsonApi\Document\Meta;
-use JsonApiPhp\JsonApi\Document\Resource\NullResource;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
@@ -91,7 +90,7 @@ public function testDocumentMayContainJustData()
                 "data": null
             }
             ',
-            Document::fromResource(new NullResource),
+            Document::nullDocument(),
             'The simplest document possible contains null'
         );
 
@@ -104,7 +103,7 @@ public function testDocumentMayContainJustData()
                 }
             }        
             ',
-            Document::fromResource(new ResourceIdentifier('books', 'abc123')),
+            Document::fromIdentifier(new ResourceIdentifier('books', 'abc123')),
             'Resource identifier can be used as primary data'
         );
 
@@ -141,7 +140,7 @@ public function testDocumentMayContainJustData()
                 ]
             }
             ',
-            Document::fromResources(
+            Document::fromIdentifiers(
                 new ResourceIdentifier('books', '12'),
                 new ResourceIdentifier('carrots', '42')
             ),
@@ -157,11 +156,11 @@ public function testDocumentMayContainJustData()
      */
     public function testDocumentCanHaveExtraProperties()
     {
-        $doc = Document::fromResource(
+        $doc = Document::fromIdentifier(
             new ResourceIdentifier('apples', '42')
         );
         $doc->setApiVersion('1.0');
-        $doc->setApiMeta(['a' => 'b']);
+        $doc->setApiMeta(Meta::fromArray(['a' => 'b']));
         $doc->setMeta(Meta::fromArray(['test' => 'test']));
         $doc->setLink('self', 'http://example.com/self');
         $doc->setLink('related', 'http://example.com/rel', ['foo' => 'bar']);
diff --git a/test/Document/Resource/Relationship/LinkageTest.php b/test/Document/Resource/Relationship/LinkageTest.php
index e71cb9e..0c7b8d9 100644
--- a/test/Document/Resource/Relationship/LinkageTest.php
+++ b/test/Document/Resource/Relationship/LinkageTest.php
@@ -11,9 +11,12 @@
 
 namespace JsonApiPhp\JsonApi\Test\Document\Resource\Relationship;
 
-use JsonApiPhp\JsonApi\Document\Resource\NullResource;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\MultiLinkage;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\NullLinkage;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Linkage;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
 /**
@@ -40,7 +43,7 @@ public function testCanCreateNullLinkage()
     {
         $this->assertEncodesTo(
             'null',
-            Linkage::nullLinkage()
+            new NullLinkage()
         );
     }
 
@@ -48,7 +51,7 @@ public function testCanCreateEmptyArrayLinkage()
     {
         $this->assertEncodesTo(
             '[]',
-            Linkage::emptyArrayLinkage()
+            new MultiLinkage()
         );
     }
 
@@ -61,7 +64,7 @@ public function testCanCreateFromSingleResourceId()
                 "id": "abc"
             }
             ',
-            Linkage::fromSingleIdentifier(new ResourceIdentifier('books', 'abc'))
+            new SingleLinkage(new ResourceIdentifier('books', 'abc'))
         );
     }
 
@@ -80,7 +83,7 @@ public function testCanCreateFromArrayOfResourceIds()
                 }
             ]
             ',
-            Linkage::fromManyIdentifiers(
+            new MultiLinkage(
                 new ResourceIdentifier('books', 'abc'),
                 new ResourceIdentifier('squirrels', '123')
             )
@@ -89,24 +92,22 @@ public function testCanCreateFromArrayOfResourceIds()
 
     public function testNullLinkageIsLinkedToNothing()
     {
-        $apple = new ResourceIdentifier('apples', '1');
-        $this->assertFalse(Linkage::nullLinkage()->isLinkedTo($apple));
-        $this->assertFalse(Linkage::nullLinkage()->isLinkedTo(new NullResource));
+        $apple = new ResourceObject('apples', '1');
+        $this->assertFalse((new NullLinkage())->isLinkedTo($apple));
     }
 
     public function testEmptyArrayLinkageIsLinkedToNothing()
     {
-        $apple = new ResourceIdentifier('apples', '1');
-        $this->assertFalse(Linkage::emptyArrayLinkage()->isLinkedTo($apple));
-        $this->assertFalse(Linkage::emptyArrayLinkage()->isLinkedTo(new NullResource));
+        $apple = new ResourceObject('apples', '1');
+        $this->assertFalse((new MultiLinkage())->isLinkedTo($apple));
     }
 
     public function testSingleLinkageIsLinkedOnlyToItself()
     {
-        $apple = new ResourceIdentifier('apples', '1');
-        $orange = new ResourceIdentifier('oranges', '1');
+        $apple = new ResourceObject('apples', '1');
+        $orange = new ResourceObject('oranges', '1');
 
-        $linkage = Linkage::fromSingleIdentifier($apple);
+        $linkage = new SingleLinkage($apple->toIdentifier());
 
         $this->assertTrue($linkage->isLinkedTo($apple));
         $this->assertFalse($linkage->isLinkedTo($orange));
@@ -114,11 +115,11 @@ public function testSingleLinkageIsLinkedOnlyToItself()
 
     public function testMultiLinkageIsLinkedOnlyToItsMembers()
     {
-        $apple = new ResourceIdentifier('apples', '1');
-        $orange = new ResourceIdentifier('oranges', '1');
-        $banana = new ResourceIdentifier('bananas', '1');
+        $apple = new ResourceObject('apples', '1');
+        $orange = new ResourceObject('oranges', '1');
+        $banana = new ResourceObject('bananas', '1');
 
-        $linkage = Linkage::fromManyIdentifiers($apple, $orange);
+        $linkage = new MultiLinkage($apple->toIdentifier(), $orange->toIdentifier());
 
         $this->assertTrue($linkage->isLinkedTo($apple));
         $this->assertTrue($linkage->isLinkedTo($orange));
diff --git a/test/Document/Resource/Relationship/RelationshipTest.php b/test/Document/Resource/Relationship/RelationshipTest.php
index 45ad99e..436384a 100644
--- a/test/Document/Resource/Relationship/RelationshipTest.php
+++ b/test/Document/Resource/Relationship/RelationshipTest.php
@@ -12,6 +12,7 @@
 namespace JsonApiPhp\JsonApi\Test\Document\Resource\Relationship;
 
 use JsonApiPhp\JsonApi\Document\Meta;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\NullLinkage;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Linkage;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
@@ -84,7 +85,7 @@ public function testCanCreateFromLinkage()
                 "data": null
             }
             ',
-            Relationship::fromLinkage(Linkage::nullLinkage())
+            Relationship::fromLinkage(new NullLinkage())
         );
     }
 
diff --git a/test/Document/Resource/ResourceTest.php b/test/Document/Resource/ResourceTest.php
index 97c0779..c1b238d 100644
--- a/test/Document/Resource/ResourceTest.php
+++ b/test/Document/Resource/ResourceTest.php
@@ -39,7 +39,7 @@ public function resourceProvider()
         return [
             [
                 '{"type": "books"}',
-                new ResourceIdentifier('books'),
+                new ResourceObject('books'),
             ],
             [
                 '{"type":"books","id":"42abc"}',
diff --git a/test/IntegrationTest.php b/test/IntegrationTest.php
index 211ed25..6eee1e5 100644
--- a/test/IntegrationTest.php
+++ b/test/IntegrationTest.php
@@ -12,7 +12,7 @@
 namespace JsonApiPhp\JsonApi\Test;
 
 use JsonApiPhp\JsonApi\Document;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Linkage;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
@@ -48,7 +48,7 @@ public function testFromTheReadmeFile()
 
         $articles = new ResourceObject('articles', '1');
         $author = Relationship::fromLinkage(
-            Linkage::fromSingleIdentifier(
+            new SingleLinkage(
                 new ResourceIdentifier('people', '9')
             )
         );