diff --git a/composer.json b/composer.json
index 8ce2d74..0decca3 100644
--- a/composer.json
+++ b/composer.json
@@ -28,6 +28,6 @@
     }
   },
   "scripts": {
-    "test": "php-cs-fixer fix -v --dry-run --ansi && phpunit --colors=always"
+    "test": "php-cs-fixer fix -v --dry-run --ansi && phpunit --colors=always --coverage-text"
   }
 }
diff --git a/src/Document/Resource/Relationship/Relationship.php b/src/Document/Resource/Relationship.php
similarity index 93%
rename from src/Document/Resource/Relationship/Relationship.php
rename to src/Document/Resource/Relationship.php
index 05b4226..49cefcb 100644
--- a/src/Document/Resource/Relationship/Relationship.php
+++ b/src/Document/Resource/Relationship.php
@@ -1,14 +1,13 @@
 <?php
 declare(strict_types=1);
 
-namespace JsonApiPhp\JsonApi\Document\Resource\Relationship;
+namespace JsonApiPhp\JsonApi\Document\Resource;
 
 use JsonApiPhp\JsonApi\Document\Link\LinkInterface;
 use JsonApiPhp\JsonApi\Document\LinksTrait;
 use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\MetaTrait;
 use JsonApiPhp\JsonApi\Document\Resource\Linkage\LinkageInterface;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 
 final class Relationship implements \JsonSerializable
 {
diff --git a/src/Document/Resource/ResourceIdentifier.php b/src/Document/Resource/ResourceIdentifier.php
index 39e354b..67eb922 100644
--- a/src/Document/Resource/ResourceIdentifier.php
+++ b/src/Document/Resource/ResourceIdentifier.php
@@ -7,8 +7,19 @@
 
 class ResourceIdentifier implements \JsonSerializable
 {
+    /**
+     * @var string
+     */
     private $type;
+
+    /**
+     * @var string
+     */
     private $id;
+
+    /**
+     * @var Meta
+     */
     private $meta;
 
     public function __construct(string $type, string $id, Meta $meta = null)
diff --git a/src/Document/Resource/ResourceObject.php b/src/Document/Resource/ResourceObject.php
index aa54892..337d65d 100644
--- a/src/Document/Resource/ResourceObject.php
+++ b/src/Document/Resource/ResourceObject.php
@@ -5,7 +5,6 @@
 
 use JsonApiPhp\JsonApi\Document\LinksTrait;
 use JsonApiPhp\JsonApi\Document\Meta;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
 
 class ResourceObject implements \JsonSerializable
 {
@@ -35,13 +34,13 @@ public function setMeta(Meta $meta)
     public function setAttribute(string $name, $value)
     {
         if ($this->isReservedName($name)) {
-            throw new \InvalidArgumentException('Can not use a reserved name');
+            throw new \InvalidArgumentException("Can not use a reserved name '$name'");
         }
         if (!$this->isValidMemberName($name)) {
-            throw new \OutOfBoundsException('Not a valid attribute name');
+            throw new \OutOfBoundsException("Not a valid attribute name '$name'");
         }
         if (isset($this->relationships[$name])) {
-            throw new \LogicException("Field $name already exists in relationships");
+            throw new \LogicException("Field '$name' already exists in relationships");
         }
         $this->attributes[$name] = $value;
     }
@@ -49,10 +48,13 @@ public function setAttribute(string $name, $value)
     public function setRelationship(string $name, Relationship $relationship)
     {
         if ($this->isReservedName($name)) {
-            throw new \InvalidArgumentException('Can not use a reserved name');
+            throw new \InvalidArgumentException("Can not use a reserved name '$name'");
+        }
+        if (!$this->isValidMemberName($name)) {
+            throw new \OutOfBoundsException("Not a valid attribute name '$name'");
         }
         if (isset($this->attributes[$name])) {
-            throw new \LogicException("Field $name already exists in attributes");
+            throw new \LogicException("Field '$name' already exists in attributes");
         }
         $this->relationships[$name] = $relationship;
     }
diff --git a/test/Document/CompoundDocumentTest.php b/test/Document/CompoundDocumentTest.php
index b22ef66..c796508 100644
--- a/test/Document/CompoundDocumentTest.php
+++ b/test/Document/CompoundDocumentTest.php
@@ -7,7 +7,7 @@
 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\Relationship;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
@@ -159,14 +159,26 @@ public function testOfficialDocsExample()
     /**
      * @expectedException \LogicException
      * @expectedExceptionMessage Full linkage is required for apples:1
+     * @dataProvider             documentsWithoutFullLinkage
+     * @param Document $doc
      */
-    public function testFullLinkageIsRequired()
+    public function testFullLinkageIsRequired(Document $doc)
     {
-        $doc = Document::nullDocument();
         $doc->setIncluded(new ResourceObject('apples', '1'));
         json_encode($doc);
     }
 
+    public function documentsWithoutFullLinkage(): array
+    {
+        return [
+            [Document::nullDocument()],
+            [Document::fromIdentifier(new ResourceIdentifier('oranges', '1'))],
+            [Document::fromIdentifiers(new ResourceIdentifier('oranges', '1'), new ResourceIdentifier('oranges', '2'))],
+            [Document::fromResource(new ResourceObject('oranges', '1'))],
+            [Document::fromResources(new ResourceObject('oranges', '1'), new ResourceObject('oranges', '1'))],
+        ];
+    }
+
     /**
      * A compound document must be explicitly marked as sparse. In this case full linkage is not required.
      */
diff --git a/test/Document/DocumentTest.php b/test/Document/DocumentTest.php
index 2d255ee..72e6336 100644
--- a/test/Document/DocumentTest.php
+++ b/test/Document/DocumentTest.php
@@ -11,7 +11,7 @@
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 use JsonApiPhp\JsonApi\Test\Document\Resource\Relationship\LinkageTest;
-use JsonApiPhp\JsonApi\Test\Document\Resource\ResourceTest;
+use JsonApiPhp\JsonApi\Test\Document\Resource\ResourceObjectTest;
 
 /**
  * This is the JSON document's top level object
@@ -68,7 +68,7 @@ public function testDocumentMayContainJustErrors()
 
     /**
      * A valid document may contain just a primary data object.
-     * The primary data object is represented by ResourceInterface (@see ResourceTest for details).
+     * The primary data object is represented by ResourceInterface (@see ResourceObjectTest for details).
      * Here is how a document can be created from different kinds of resources:
      * - null resource
      * - resource identifier
diff --git a/test/Document/Resource/Relationship/RelationshipTest.php b/test/Document/Resource/Relationship/RelationshipTest.php
index 8b98734..6fdfe10 100644
--- a/test/Document/Resource/Relationship/RelationshipTest.php
+++ b/test/Document/Resource/Relationship/RelationshipTest.php
@@ -6,8 +6,8 @@
 use JsonApiPhp\JsonApi\Document\Link\Link;
 use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\Resource\Linkage\NullLinkage;
+use JsonApiPhp\JsonApi\Document\Resource\Relationship;
 use JsonApiPhp\JsonApi\Document\Resource\Relationship\Linkage;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
 use JsonApiPhp\JsonApi\Test\BaseTestCase;
 
 /**
@@ -52,7 +52,7 @@ public function testCanCreateFromSelfLink()
                 }
             }
             ',
-            Relationship::fromSelfLink(new Link('http://localhost'))
+            \JsonApiPhp\JsonApi\Document\Resource\Relationship::fromSelfLink(new Link('http://localhost'))
         );
     }
 
@@ -66,7 +66,7 @@ public function testCanCreateFromRelatedLink()
                 }
             }        
            ',
-            Relationship::fromRelatedLink(new Link('http://localhost'))
+            \JsonApiPhp\JsonApi\Document\Resource\Relationship::fromRelatedLink(new Link('http://localhost'))
         );
     }
 
@@ -78,7 +78,7 @@ public function testCanCreateFromLinkage()
                 "data": null
             }
             ',
-            Relationship::fromLinkage(new NullLinkage())
+            \JsonApiPhp\JsonApi\Document\Resource\Relationship::fromLinkage(new NullLinkage())
         );
     }
 
diff --git a/test/Document/Resource/ResourceFieldsTest.php b/test/Document/Resource/ResourceFieldsTest.php
index c301fb3..f8aede2 100644
--- a/test/Document/Resource/ResourceFieldsTest.php
+++ b/test/Document/Resource/ResourceFieldsTest.php
@@ -4,7 +4,7 @@
 namespace JsonApiPhp\JsonApi\Test\Document\Resource;
 
 use JsonApiPhp\JsonApi\Document\Meta;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
+use JsonApiPhp\JsonApi\Document\Resource\Relationship;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use PHPUnit\Framework\TestCase;
 
@@ -23,7 +23,7 @@ class ResourceFieldsTest extends TestCase
 {
     /**
      * @expectedException \LogicException
-     * @expectedExceptionMessage Field foo already exists in attributes
+     * @expectedExceptionMessage Field 'foo' already exists in attributes
      */
     public function testCanNotSetRelationshipIfAttributeExists()
     {
@@ -34,7 +34,7 @@ public function testCanNotSetRelationshipIfAttributeExists()
 
     /**
      * @expectedException \LogicException
-     * @expectedExceptionMessage Field foo already exists in relationships
+     * @expectedExceptionMessage Field 'foo' already exists in relationships
      */
     public function testCanNotSetAttributeIfRelationshipExists()
     {
@@ -67,29 +67,6 @@ public function testRelationshipCanNotHaveReservedNames(string $name)
         $res->setRelationship($name, Relationship::fromMeta(Meta::fromArray(['a' => 'b'])));
     }
 
-    /**
-     * @param string $name
-     * @expectedException \OutOfBoundsException
-     * @expectedExceptionMessage Not a valid attribute name
-     * @dataProvider             invalidAttributeNames
-     */
-    public function testAttributeNameIsNotValid(string $name)
-    {
-        $res = new ResourceObject('books', 'abc');
-        $res->setAttribute($name, 1);
-    }
-
-    /**
-     * @param string $name
-     * @dataProvider             validAttributeNames
-     */
-    public function testAttributeNameIsValid(string $name)
-    {
-        $res = new ResourceObject('books', 'abc');
-        $res->setAttribute($name, 1);
-        $this->assertTrue(true);
-    }
-
     public function reservedAttributeNames(): array
     {
         return [
@@ -97,35 +74,4 @@ public function reservedAttributeNames(): array
             ['type'],
         ];
     }
-
-    public function invalidAttributeNames(): array
-    {
-        return [
-            ['_abcde'],
-            ['abcd_'],
-            ['abc$EDS'],
-            ['#abcde'],
-            ['abcde('],
-            ['b_'],
-            ['_a'],
-            ['$ab_c-d'],
-            ['-abc'],
-        ];
-    }
-
-    public function validAttributeNames(): array
-    {
-        return [
-            ['abcd'],
-            ['abcA4C'],
-            ['abc_d3f45'],
-            ['abd_eca'],
-            ['a'],
-            ['b'],
-            ['ab'],
-            ['a-bc_de'],
-            ['abcéêçèÇ_n'],
-            ['abc 汉字 abc'],
-        ];
-    }
 }
diff --git a/test/Document/Resource/ResourceIdentifierTest.php b/test/Document/Resource/ResourceIdentifierTest.php
new file mode 100644
index 0000000..bc1c7f0
--- /dev/null
+++ b/test/Document/Resource/ResourceIdentifierTest.php
@@ -0,0 +1,45 @@
+<?php
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Test\Document\Resource;
+
+use JsonApiPhp\JsonApi\Document\Meta;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Test\BaseTestCase;
+
+/**
+ * Resource Identifiers
+ *
+ * @see http://jsonapi.org/format/#document-resource-identifier-objects
+ */
+class ResourceIdentifierTest extends BaseTestCase
+{
+    public function testResourceIdentifierMustContainTypeAndId()
+    {
+        $this->assertEncodesTo(
+            '
+            {
+                "type": "books",
+                "id": "1"
+            }
+            ',
+            new ResourceIdentifier('books', '1')
+        );
+    }
+
+    public function testResourceIdentifierMayContainMeta()
+    {
+        $this->assertEncodesTo(
+            '
+            {
+                "type": "books",
+                "id": "1",
+                "meta": {
+                    "foo":"bar"
+                }
+            }
+            ',
+            new ResourceIdentifier('books', '1', Meta::fromArray(['foo' => 'bar']))
+        );
+    }
+}
diff --git a/test/Document/Resource/ResourceObjectTest.php b/test/Document/Resource/ResourceObjectTest.php
new file mode 100644
index 0000000..8dfdb37
--- /dev/null
+++ b/test/Document/Resource/ResourceObjectTest.php
@@ -0,0 +1,111 @@
+<?php
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Test\Document\Resource;
+
+use JsonApiPhp\JsonApi\Document\Meta;
+use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;
+use JsonApiPhp\JsonApi\Document\Resource\Relationship;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+use JsonApiPhp\JsonApi\Test\BaseTestCase;
+
+/**
+ * Resource Objects
+ *
+ * @see http://jsonapi.org/format/#document-resource-objects
+ */
+class ResourceObjectTest extends BaseTestCase
+{
+    /**
+     * The id member is not required when the resource object originates at the client
+     * and represents a new resource to be created on the server.
+     */
+    public function testResourceObjectIdIsOptional()
+    {
+        $this->assertEncodesTo('{"type": "books"}', new ResourceObject('books'));
+    }
+
+    /**
+     * In addition, a resource object MAY contain any of these top-level members:
+     *
+     * - attributes: an attributes object representing some of the resource’s data.
+     *
+     * - relationships: a relationships object describing relationships
+     *   between the resource and other JSON API resources.
+     *
+     * - links: a links object containing links related to the resource.
+     *
+     * - meta: a meta object containing non-standard meta-information about a resource
+     *   that can not be represented as an attribute or relationship.
+     */
+    public function testResourceObjectMayContainAttributes()
+    {
+        $apple = new ResourceObject('apples', '1');
+        $apple->setAttribute('color', 'red');
+        $this->assertEncodesTo('{"type":"apples", "id":"1", "attributes":{"color":"red"}}', $apple);
+    }
+
+    public function testResourceObjectMayContainRelationships()
+    {
+        $article = new ResourceObject('articles', '1');
+        $user = new ResourceIdentifier('users', '42');
+        $article->setRelationship('author', Relationship::fromLinkage(new SingleLinkage($user)));
+        $this->assertEncodesTo(
+            '
+            {
+                "type":"articles", 
+                "id":"1",
+                "relationships":{
+                    "author":{
+                        "data":{
+                            "type":"users",
+                            "id":"42"
+                        }
+                    }
+                }
+            }
+            ',
+            $article
+        );
+    }
+
+    public function testResourceObjectMayContainLinks()
+    {
+        $article = new ResourceObject('articles', '1');
+        $article->setLink('self', 'https://example.com');
+        $this->assertEncodesTo(
+            '
+            {
+                "type":"articles", 
+                "id":"1",
+                "links":{
+                    "self":"https://example.com"
+                }
+            }
+            ',
+            $article
+        );
+    }
+
+    public function testResourceObjectMayContainMeta()
+    {
+        $article = new ResourceObject('articles', '1');
+        $article->setMeta(Meta::fromArray(['tags' => ['cool', 'new']]));
+        $this->assertEncodesTo(
+            '
+            {
+                "type":"articles", 
+                "id":"1",
+                "meta":{
+                    "tags":[
+                        "cool",
+                        "new"
+                    ]
+                }
+            }
+            ',
+            $article
+        );
+    }
+}
diff --git a/test/Document/Resource/ResourceTest.php b/test/Document/Resource/ResourceTest.php
deleted file mode 100644
index eeb3e7e..0000000
--- a/test/Document/Resource/ResourceTest.php
+++ /dev/null
@@ -1,86 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace JsonApiPhp\JsonApi\Test\Document\Resource;
-
-use JsonApiPhp\JsonApi\Document\Meta;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
-use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
-use JsonApiPhp\JsonApi\Test\BaseTestCase;
-
-/**
- * Resource Objects
- *
- * @see http://jsonapi.org/format/#document-resource-objects
- */
-class ResourceTest extends BaseTestCase
-{
-    /**
-     * @param string $expected
-     * @param mixed  $data
-     * @dataProvider resourceProvider
-     */
-    public function testSerialization(string $expected, $data)
-    {
-        $this->assertEncodesTo($expected, $data);
-    }
-
-    public function resourceProvider()
-    {
-        return [
-            [
-                '{"type": "books"}',
-                new ResourceObject('books'),
-            ],
-            [
-                '{"type":"books","id":"42abc"}',
-                new ResourceIdentifier('books', '42abc'),
-            ],
-            [
-                '
-                {
-                    "type": "books",
-                    "id": "42abc",
-                    "meta": {
-                        "foo":"bar"
-                    }
-                }
-                ',
-                new ResourceIdentifier('books', '42abc', Meta::fromArray(['foo' => 'bar'])),
-            ],
-            [
-                '
-                {
-                    "type": "books",
-                    "id": "42abc",
-                    "attributes": {
-                        "attr": "val"
-                    },
-                    "relationships": {
-                        "author": {
-                            "meta": {
-                                "a": "b"
-                            }
-                        }
-                    },
-                    "links": {
-                        "self": "http://localhost"
-                    },
-                    "meta": {
-                        "foo": "bar"
-                    }
-                }
-                ',
-                (function () {
-                    $resource = new ResourceObject('books', '42abc');
-                    $resource->setMeta(Meta::fromArray(['foo' => 'bar']));
-                    $resource->setAttribute('attr', 'val');
-                    $resource->setLink('self', 'http://localhost');
-                    $resource->setRelationship('author', Relationship::fromMeta(Meta::fromArray(['a' => 'b'])));
-                    return $resource;
-                })(),
-            ],
-        ];
-    }
-}
diff --git a/test/IntegrationTest.php b/test/IntegrationTest.php
index 12b189d..a884d86 100644
--- a/test/IntegrationTest.php
+++ b/test/IntegrationTest.php
@@ -5,7 +5,7 @@
 
 use JsonApiPhp\JsonApi\Document;
 use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;
-use JsonApiPhp\JsonApi\Document\Resource\Relationship\Relationship;
+use JsonApiPhp\JsonApi\Document\Resource\Relationship;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceIdentifier;
 use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
 use PHPUnit\Framework\TestCase;
diff --git a/test/MemberNamesTest.php b/test/MemberNamesTest.php
new file mode 100644
index 0000000..d7c405f
--- /dev/null
+++ b/test/MemberNamesTest.php
@@ -0,0 +1,89 @@
+<?php
+declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Test;
+
+use JsonApiPhp\JsonApi\Document\Link\Link;
+use JsonApiPhp\JsonApi\Document\Resource\Relationship;
+use JsonApiPhp\JsonApi\Document\Resource\ResourceObject;
+use PHPUnit\Framework\TestCase;
+
+class MemberNamesTest extends TestCase
+{
+    /**
+     * @param string $name
+     * @expectedException \OutOfBoundsException
+     * @expectedExceptionMessage Not a valid attribute name
+     * @dataProvider             invalidAttributeNames
+     */
+    public function testInvalidAttributeNamesAreNotAllowed(string $name)
+    {
+        $res = new ResourceObject('books', 'abc');
+        $res->setAttribute($name, 1);
+    }
+
+    /**
+     * @param string $name
+     * @dataProvider             validAttributeNames
+     */
+    public function testValidAttributeNamesCanBeSet(string $name)
+    {
+        $res = new ResourceObject('books', 'abc');
+        $res->setAttribute($name, 1);
+        $this->assertInternalType('string', json_encode($res));
+    }
+
+    /**
+     * @param string $name
+     * @expectedException \OutOfBoundsException
+     * @expectedExceptionMessage Not a valid attribute name
+     * @dataProvider             invalidAttributeNames
+     */
+    public function testInvalidRelationshipNamesAreNotAllowed(string $name)
+    {
+        $res = new ResourceObject('books', 'abc');
+        $res->setRelationship($name, Relationship::fromSelfLink(new Link('https://example.com')));
+    }
+
+    /**
+     * @param string $name
+     * @dataProvider             validAttributeNames
+     */
+    public function testValidRelationshipNamesCanBeSet(string $name)
+    {
+        $res = new ResourceObject('books', 'abc');
+        $res->setRelationship($name, Relationship::fromSelfLink(new Link('https://example.com')));
+        $this->assertInternalType('string', json_encode($res));
+    }
+
+    public function invalidAttributeNames(): array
+    {
+        return [
+            ['_abcde'],
+            ['abcd_'],
+            ['abc$EDS'],
+            ['#abcde'],
+            ['abcde('],
+            ['b_'],
+            ['_a'],
+            ['$ab_c-d'],
+            ['-abc'],
+        ];
+    }
+
+    public function validAttributeNames(): array
+    {
+        return [
+            ['abcd'],
+            ['abcA4C'],
+            ['abc_d3f45'],
+            ['abd_eca'],
+            ['a'],
+            ['b'],
+            ['ab'],
+            ['a-bc_de'],
+            ['abcéêçèÇ_n'],
+            ['abc 汉字 abc'],
+        ];
+    }
+}